My tooth hurts, and I'm bored...time to do some cast analysis in terms of whose actually in the game. I am not analyzing based off of character quality or anything, more just "who got in" and "how reasonable is their inclusion" followed by proposing possible "reasonable" additions and how they might appear.
The characters can be split up into 5 main categories I feel:
Permanent character: Straight forward; character can be gotten permanently, be it optional or required. These characters are the ones who don't really need an analysis as to "why they're in" for obvious reasons. I will, however, highlight spin-off characters.
Meaningful Temp: Lack of a better word than "meaningful", this refers to a character who spends a significant amount of time in the team and is not restricted in anyway from the primary gameplay mechanics. For example, using FF8, a primary gameplay mechanics would be stuff like Junctioning, where as Limit Break progression does not really qualify. Like Permanent characters, Meaningful Temps feel equivalent in the sense of being expected additions, and don't really need justification.
Minor Temp: Character is technically playable, but doesn't last a long time. These are the ones I'll actually be analyzing genearlly
Bosses: The character is a boss. This can be put in conjunction with a previous one, to be clear. Like minor Temps, this I'll be going into more depth.
Miscellaneous: Character that doesnt' qualify any of the above, and thus must be analyzed "Why."
Without further ado, here we go!
Final Fantasy 1:
Warrior of Light qualifies as a permanent PC, so self explanatory. Garland is a boss, and he mostly got in because he's the closest thing FF1 has to a main villain so who else you going to use. The other 3 are all miscellaneous, so...
Wol: FF1 is starved for character options, and from my understanding, Moebius is a re-imagining of FF1, so making Wol an honorary FF1 character is something you can get away with. I don't think they'd attempt this stunt with any other game, it's really just "FF1 has that few options." he's the main PC of MFF
Ecco: Like Wol, but from my understanding, she's basically Navi of the game? This makes her the primary NPC I guess? I heard she does assist you in battle, which is about as good as you'll get, so again, like Wol, it was out of necessity.
Sarah: To make a full roster of 5 characters, they really had to dig the bottom of the barrel, and Sarah WAS playable in Theatrhythm, and probably the most memorable NPC if only because the first one who does anything and the Lute does give her a cheap excuse for bard and...yeah, it's "FF1 has no good ideas."
POTENTIAL ADDITIONS:
Matoya: Is a popular NPC by FF1 standards, seeing as Ecco was turned into a Sage, they could probably make Matoya a Black mage/Summoner, but only if she has a Soul Break where she goes Sorcerer's Apprentice on the enemies ass...bonus points if Mickey is involved! Which is to say, she's perfect for a Kingdom Hearts Collaboration Event! *shot*
Final Fantasy 2:
The entire roster here is made of up Permanent PCs, OR Meaningful Temps (everyone else). No need to really go into detail from there, since they're all self explanatory.
POTENTIAL ADDITIONS:
Emperor: Basically an Edea variant, having Black 5 and Darkness 5. yeah, he's boring, but that's really the best I got. Maybe you can get away with Spellblade to cover the wind elemental attacks for the Tornado, as well as his draining Physical.
Scott: I imagine a Celes-style build of being a legitimate Red Mage, though clearly leaning towards the physical end. He wouldn't have her insane res, but he'd have Black 5 (Wild Rose is a heavy Fighter/Mage weapon) and Combat 4, in exchange for only Spellblade 3/4. This best reflects his appearance in Soul of Rebirth where he's got full mithril gear, including a sword, but also several elemental spells and healing ones, and his weapon as I noted is built around physical and magic stats, a rare combo.
Final Fantasy 3:
All but Desch fall under "Permanent PCs" and thus no need to get into that.
Desch falls under "Miscellaneous" or "Minor Temp" depending on how you view it. Similar to the entire FF1 roster save Warrior of Light, he was basically implemented out of necessity more than anything else. Sounds like Onion Knight is coming soon being some unique build, which will be weird having a core-like character (in that he's named after a job) attached to a realm, but we'll see.
OTHER POTENTIAL IDEAS:
Cloud of Darkness: Give her Darkness 5 because villain, and...I don't want to be boring and give her Black 5 but I feel that's almost inevitable. Her stats...part of me wants to say Vincent-like so she can emphasize both forms of offense, another side of me says "you moron, she was stupidly durable in FF3DS, give her massive HP!" and yeah, I got nothing.
Everyone else is going to be a Desch style character, because he set a precedent so...
Doga: Sage build, this seems straight forward!
Unei: Just to make her different from Doga, I'll make her the primary Black Mage because Desch sucks
Cid: Some form of Machinist; I honestly can't care enough beyond that
Alus, Aria and Sarah invoke even more apathy than the above characters, so erally don't care there. HOWEVER, there is one I left off...
Old Man of Amur: Gilgamesh level physical stats, Celes' res, Shantotto's Magic, and Tyro's equipment/skillset. HE IS THE TRUE HERO OF FF3! AND THERE'S 4 OF HIM!
Final Fantasy 4:
Cecil, Rosa, Rydia, Kain and Edge are permanent characters, straight forward. Palom, Porom, Edward, and Yang are Meaningful Temps AND Permanent PCs depending on the version, they make sense. Tellah and FuSoYa are just straight up meaningful temps, and might as well add D. Cecil there technically. Golbez is a lone boss fight, but he's also the game's main villain for a majority of the game so he's perfectly acceptable, with a build that strongly resembles his TAY form, which makes sense. So everyone makes sense here.
Except there's one blatant omission that still feels weird, especially after they JUST had an FF4 event to put him in:
Cid: A Meaningful Temp/Permanent PC based on version, it's weird he's still absent. Give him Combat 5, Machinist 5, Spellblade 4 (Augment in FF4DS is reminiscent of this and FF4 lacks a Spellblade to boot, so it works well), and that should work. He can use all defensive gear except Robes like most meleers (part of me wants to say "no" to Shields due to two handed weapons but that's straight up wrong since he CAN use 1H Axes), weapons would be Daggers, Axes, Hammers, and Bows.
Also, because spin off/sequels are on the table, might as well address the Not!Fanfic in the room in TAY and look at some of those characters, only picking a handful:
Ceodore: A Paladin but with actual stats to run as a primary healer, instead of "is a Knight with Shellga/Protectga/Dispel/Reflect!" and lower physical stats to compensate. He'd get Lances but no Axes as well.
Ursula: I think of her as a monk trading in HP for Celerity 5, making her a Chainstarter + Full Charge Monk. It easily distinguishes her from her father at least.
Luka: Cid's equipment, skillset would be Support 5, Machinist 5, Shooter 4.
Leonora: White 5, Black 4...because we totally don't have enough mages in this realm or anything!
And sadly, this would result in Kain getting White 4 because fuck Holy Dragoon bullshit.
Final Fantasy 5:
Bartz, Lenna, Krile and Faris are Permanent PCs, Galuf is a Meaningful Temp, Ex-death is the game's main antagonist, and Gilgamesh is a recurring boss you fight a lot. No real stand out oddities here.
POTENTIAL ADDITIONS:
the only thing I can think that could be added to FF5...not that they need anything more because the cast is ridiculous as is but I digress...would be a few NPCs that feel almost Braska-level (we'll get to him!) So without further ado...
Dorgan: Give him a bunch of physical stuff, not sure what but he's clearly a swordsmen of some kind.
Kelgar: Celerity 5, Combat 5, Ninja 4. He's clearly fast so Celerity is a given, and being a wolf I imagine he's strong so let's make him Chain starter + Full Charge combo. Ninja because the way he moves has after images and such and look, I'm trying my best ok?
Zezat: Spellblade 5, Knight 5,one of Support, Celerity or Combat at like 4. He's a swordsmen and he clearly uses a lightning attack somewhere so yeah, Spellblade it is. Knight 5 just feels right because he's a king or something, and he needs some generic skillset, so take your pick!
Ghido: Celerity 5 because he's really fast and having a TURTLE WITH CELERITY 5 IS HILARIOUSLY IRONIC. Knight 5 as well because that's a tank skillset and HE"S A FREAKING TURTLE. Also give him some magic levels because he's smart, and make a new skillset called Mock Idiots and give him 7* in that, which powers him up anytime someone classified as an idiot is in the team.
King Tycoon: Some kind of mage, I don't care.
No, I don't expect any of those to get in, but thought I'd toss the Warriors of Dawn in, King Tycoon since he plays a notable role in World 1, and Ghido because we need someone to get stronger when put in a team of Bartz, Wakka, Yda, and Gilgamesh.
Final Fantasy 6:
Kefka aside, everyone is a Permanent PC, with Kefka of course being the game's main villain thus an obvious entry.
We lack Umaro and Gogo, but Umaro I feel would be hard to implement appropriately without pulling a D. Cecil scenario which would be lame, so I suspect he'll be relegated to Mog's BSB which would all be attacks involving Umaro using him as a ballistic missile.
Gogo on the otherhand? Give him 4* in everything, stats superior to Tyro, but he's restricted to Daggers, Rods, Staves, Whips (Scorpion Tail is his DD weapon, so yeah), maybe instruments because why not, Hats, Robes, Armlets, Shields, and MAYBE Light Armor. The idea obviously is a variation of Tyro, trading in some diversity (since that's Tyro's thing) for having "stats that are better than a Core." Note that Gogo's stats wouldn't be particularly stand out, of course, they just would qualify as a level beyond "sucks."
OTHER ADDITIONS:
Leo: Shear demand suggests people want him in. Combat 5 and Knight 5 are a given. Celerity 4 feels appropriate because need to show his speed and offering in some manner, and some level of Spellblade to represent Shock (I guess 3* would fit.) His equipment set should mirror Steiner's, since while yes, he only uses Swords in FF6, I feel like something such as Axes would fit him too.
Banon: White 5, and that's it; Banon sucks and should be obsoleted by Relm in everyway.
? ? ? ? ? ?: Can't use any equipment or weapon, but his Soul Break is instant death to ANYTHING at the cost of his life. I am totally aware of all the problems with this, just damn it, let me have fun!
Final Fantasy 7:
Cloud, Tifa, Barret, Vincent, Yuffie, Red XIII, Cid, and Cait Sith are permanent PCs, Aerith is a Meaningful Temp, self explanatory there. Sephiroth is the game's Main VIllain, so boss that needs no explanation.
In respects to FF7, Zack is an extremely odd choice, but obviously they were building him off Crisis Core, so his inclusion isn't so weird when you factor that in.
Reno you fight several times...and is still an oddball despite that. See, in FF5's case, Gilgamesh you fight not only more times than Reno, but most of his encounters are required and FF5 has a much smaller roster, with 5 PCs and a Villain otherwise. FF7, we have 9 PCs, a Villain, and a protagonist of a major spin off; it begs the question of "Why did we need Reno?" Yeah, he was FF7's first Black Mage, but Vincent popped up later anyway, and did Reno's schtick of "Fighter/Mage" a hell of a lot better (though Reno can now Lifesiphon so there's that I guess.) He remains among the highest on the "oddball" picks as a result, especially since he was prioritized over Cait Sith and Vincent.
OTHER ADDITIONS:
...none, really. Granted, this is RK and I'm sure DeNA will find someway to implement the likes of Hojo, Rufus, Godo, Don Corneo, the Honey Bee Inn Gay Wrestlers, but beyond that, I don't think anyone from FF7 really should be added in.
Well, the one exception I guess you might be able to do is put Genesis from Crisis Core, with a build of Knight 5 (feels like a CHAMPION OF THE GODDESS), Darkness 5 (villain), Combat 5 (generic skillset), and Bard 3 (gotta implement Loveless SOMEHOW.) Not saying they should, but Genesis feels like the only reasonable choice left I could see them doing that wouldn't feel quite forced.
Final Fantasy 8:
Squall, Irvine, Rinoa, Selphie, Zell and Quistis are the permanent PCs, Seifer and Edea are both major bosses AND Meaningful Temps, and Laguna is a meaningful temp. No really oddities here in sense of inclusion.
POSSIBLE ADDITIONS:
Kiros: FF8 lacks a thief AND ninja, and he'd fit both pretty well considering his style, so give him Thief 5, Ninja 4, and Celerity 5. Weapons would be Claws and Katanas I guess.
Ward: FF8 also lacks a Dragoon, and using a giant anchor, he'd fit that. Combat 5, Dragoon 5, Knight 3 (because he's a big tough guy, Draw Fire seems to fit), can use Lances and Throwing Weapons.
You could maybe twist my arm with Fujin and Raijin, but there's little about them that makes them any better than Reno so...
Final Fantasy 9:
Kuja is a boss and the main antagonist, Beatrix is a recurring boss whose also a Minor Temp (if a bad ass one), the others are all permanent PCs, so the choices make sense.
POSSIBLE ADDITIONS:
Marcus: The most longstanding Temp in FF9, I feel he's worth singling out. Unfortunately, I can't really think of a way to differentiate him that would keep him as something other than "Worse Zidane"; normally I'd say "he can use Swords" but Zidane can do that too...I guess he can use Axes or something. Celerity 5, Thief 5, who the fuck cares what else.
No one else in FF9 really feels appropriate, either because redundant like Blank and Cinna, or "shut up fanboys, that character is worse than Reno in every possible way and it's by a wide margin to boot" like Lani.
Final Fantasy 10:
Everyone but Braska and Jecht are permanent PCs. Jecht is a major boss fight who is SORT OF the game's main antagonist, plus Dissidia to his name, so he fits. Braska is an oddity, though extremely important to FF10's Lore, and enough to work with that you can bullshit a skillset out of; one of the odder picks, to be sure.
Paine deserves mention since she's a Sequel-only character, but a permanent PC in said game, so her inclusion makes sense, not to mention she covers two niches FF10 was lacking in Knight and Spellblade.
POTENTIAL ADDITIONS:
Seymour: High on the "why isn't he in the game yet?" list, especially when we have Braska running around. He'd be Black 5, Summon 5, White 3, Darkness 5. Most of that is self explanatory, but White 3 is there because he knows some White Magic in FF10, but I also clearly wanted him to have a deficiency, skillset wise, compared to Yuna and Braska, rather than look straight up more appealing than both, so white 3 feels like a healthy compromise.
Final Fantasy 11:
Shantotto's the only one in and she's...a gag NPC who has her own mini story and yeah, she got in thanks to Grandfather clause and seniority.
So I'm just going to list a bunch of characters I feel SHOULD get in, based off watching cutscenes and taking a shot everytime a new apostrophe name appears.
Lion: One of the primary NPCs of the Rise of Zilart/Shadow Lord saga (I hesitate to call her "Protagonist" because she doesn't feel central), she'd have a typical Thief build from what I saw.
Zeid: Another major NPC in the same storyline, apparently plays a notable role in others. He'd be your typical Dark Knight build ala Leon and Seifer
Aldo: He was a major character but **** if I could get a read on his fighting style; just listing him for completion.
Prishe: Probably the next FF11er coming just based off history, she also makes sense being functionally the protagonist of the Chains of Promathia storyline which is easily the biggest and most popular storyline from my understanding. She'd have a typical Monk build w/ White 4 tacted on.
Tenzen: Notable NPC in Chains of Promathia, he'd have Combat 5, Samurai 5, and probably somethign else to make him not suck; I was going to give him Summon because Phoenix but Locke already shows you can handle that with just Soul Breaks.
Ulmia: White Mage/Bard build; she's a notable NPC, not much else to say
The Chebukki Triplets: Go-Die-In-A-Ditch 8*. I hate these bastards that much.
Louverance: Knight 5, Thief 4, don't care what else.
...and now I've confused most people here, MWAHAHAHAAHH!
Final Fantasy 12:
The 6 permanent PCs are in, Gabranth is the "go to" Villain rep thanks to Dissidia, and being a judge is iconic of FF12, as well as a major boss figure, so he gets a pass, and Larsa's a minor temp who plays a notable role as an NPC.
OTHER POTENTIALS:
Vossler: He's a notable minor temp, but I can't be bothered to think of a unique set for him
Reddas: ...ditto this guy
Al-Cid: Because shut up, he's awesome! I'm sure there's some set up that fits him based off his presentation in FFTA2
Final Fantasy 13:
All 6 main PCs are there, Serah is basically the same situation as Zack being an NPC (if a more notable one) in the main game, but protagonist of a spin off, and Noel is basically a Paine situation of major PC in the sequel, thus honorary member of the roster. That leaves Cid Raines, who is basically the closest thing FF13 has to a usable main antagonist...well, they COULD use EVIL MECHA POPE in human form and...wait why didn't they actually? Still, Raines is a notable face throughout FF13, so he works.
POTENTIAL ADDITIONS:
Dysley: Like I said, why the hell wasn't this guy used? White 5 and Darkness 5, making him a Darkness White Mage...no I don't think that would work, but it fits thematically damn it!
Caius: He's basically the only spin-off villain of a mainline game to be notable in any sense (outside of maybe Genesis), so I figure it couldn't hurt to bring him up here. Some Magic Knight build w/ Darkness 5 is what I see.
Final Fantasy 14:
The 4 main Scions are in, which is to be expected I suppose.
So Characters who SHOULD get in just like FF11, only this time I feel more confident since I actually, you know, played FF14.
Alphinaud: He's basically the game's protagonist all things considered *punts the 'Y'shtola as primary rep' nonsense*, and easily the biggest NPC in the game and unlike Minfilia, the one people were saying beforehand (not unfound in respects to 2.X to be fair), Alphinaud actually fights and isn't a running gag at lacking combat ability. He'd be another White Mage/Summoner hybrid, maybe get some Black Magic, and be someone who could use books besides Tyro.
Cid Garlond: One of the more prominent Cid's in the series and one who can actually fight too, he can use Swords, Guns, Machinist 5, Knight 4 (he's a Gladiator so I feel Knight 4 is a decent compromise), and Support 5
Estinien: Is a notable NPC throughout all of HW, and he'd be a Dragoon...hell, just copy and paste Kain outside of sword usage and you'd be accurate
Ysayle: Feels like she should be in and I think she's a Thaumaturge so...something with Black 5, maybe Summon levels because Shva
Haurchefant: Combat 5, Knight 5, yes this is boring but it's basically all that fits him.
Rauhbahn: Combat 5, Monk 4, Samurai 4, Knight 5; yeah not too dissimilar to Gilgamesh but feels accurate. Also he's one of the main City State leaders
Merlwyb: Support 5, Shooter 5, and some tanky set up because she's a Roegadyn
Kan-E-Senna: White 5, and really anything else
Ser Ayemeric: Knight 5, White 4, Combat 5; typical Paladin build, etc.
Hildibrand: Gentleman 5. Yes, they will create an entire skillset specifically for him; others can use it but that'll be a post for another time.
Godbert Manderville: See the Oldmen in FF3? Yeah, make those 5* into 10*, and double the stats. NOTHING ELSE WILL SUFFICE!
Probably some villains but I'm too lazy to handle them. I would put Nanamo in, but frankly, she'd work better as just a powered up state for Rauhbahn by sitting on his shoulder.
Final Fantasy Tactics:
Ramza, Agrias and Mustadio are all permanent PCs, Delita and Ovelia are minor temps but major NPCs...odd they were prioritized over other Permanent PCs, but let's go with the other permanent PCs to handle them!
OTHER ADDITIONS yada yada yada:
Rafa: Jokes aside, she could probably be some sort of Sage build.
Malak: Like her sister, a Sage build, lower magic but give him game best Magic Defense, even higher than Celes
Beowulf: Knight 4, Spellblade 3, Support 5, Black 3 because he's gotta have magic somewhere
Reis: Monk 5, Summon 3, White 4, Black 4...yeah, she's hard to really do well; I was going to say Dragoon but that's really just nothing but Jumps and thus is lame
Orlandu: Knight 5, Combat 5, Darkness 4, Spellblade 5, Support 3. Knight and Combat are givens, Darkness is only at 4 because while he does have Dark Knight skills, FFTWotL shows there's more to Dark knight than 2 skills, so let's say Darkness 4 is a good compromise, Spellblade 5 because if Delita gets Spellblade 5, Orlandu sure as hell should too. Support 3 is chosen because it demonstrates his ability to hit status, but doesn't give him Breakdowns; yes he can break equipment, but he can't break stats without using Battle Skills, so I feel letting him have Stat breaks via Combat but not Breakdowns via support is a fair compromise. Also speed is not a stat he can hit which is why I didn't give him Celerity 3. Yes, Orlandu should be strong, but we're trying to not make him stupidly OP for the sake of keeping RK interesting. On top of that, no, he shouldn't get Samurai because none of his skillset remotely resembles Samurai, and plenty of characters like Vaan, Zack, and Bartz demonstrate usage of Katanas without any Samurai levels (though I believe the reverse is true that all users of Samurai can use Katanas)
Meliadoul: Combat 5, Knight 5...yeah, not much else you can do with her. She'd get access to Bows and Lances as her one niche over Orlandu which yes sucks but it's accurate to FFT at least!
Worker 8: Do not put in, just make Ramza have a Soul Break where-in Worker 8 dances, then explodes the person...hell, make that his OSB!
Byblos: Not even worthy of being a Soul Break, he's just lame.
...no I didn't forget Rad, Alicia and Lavian, they just don't count and should be cores.