Might as well start this up! I played on Conquest, so for now all thoughts are based on that route, but some of it will generalise to Birthright reasonably well. (The specifics on equipment and character classes will be different, of course.)
Some interp stuff to discuss:
Classes and class changing
At base it's kinda like Awakening. Unpromoted PCs use Master Seals to promote, and there are (usually) two choices. Additionally, everyone has access to a second class branch, in addition to the one they start on. (They can get access to more, but it requires supports, and is presumably not DL legal.) A Heart Seal can be used to change classes. Unlike in Awakening, changing classes does not do anything to change or lower your level. If Elise uses a Heart Seal as a Level 15 Troubadour, she can become a Level 15 Wyvern Rider.
Skills are gained at 1 and 10 on unpromoted classes, then 5 and 15 on promoted ones. If you gain a level in a class for which you don't yet have the skills (due to reclassing), you gain the lowest-level skill you don't yet have. This includes skills from prerequisite classes.
What's legal, here? One option is to just allow unpromoted people to promote, and promoted folks are stuck in whatever class they're in. That's closer to how past FEs did it, but deprives the prepromos of a valuable choice which unpromoted characters get.
Another, which I favour a bit more, is to allow everyone to use one Seal. For unpromoted types this will always be a Master Seal, for prepromos it will be a Heart Seal. I'm not sure what should be seen as legal for reclassing. Only the other class on their primary class tree? (e.g. Great Knight/Paladin) Any class they have access to? Some of the prepromos really won't want this option anyway.
Still another option is to assign characters a budget and let them buy as many seals as they want, but take it out of their equipment budget. Since everyone has access to only 2-3 class branches, this does keep things rather unique.
There's also a question of if PCs are locked into a class, or if they can choose fight-to-fight.
Levels
Stat topic level will be 20/15, because it's by far the nicest. (I also finiished around there.) Reasons to make it this are that 15 promoted is when each class gains its final skill, so going lower means we miss out on those. Going higher means a few characters ram their level cap, which isn't terribly fair to them.
Special cases:
-Jakob and Felicia gain levels as if they are 20 levels lower than displayed, and cap at 40. Thus, their DL level is 35.
-Gunther gains levels as if he is 15 levels lower than displayed, and caps at 30. Thus, his DL level is 30.
-Camilla (and, from what I've read, Reina) gain levels as if she is 5 levels lower than displayed. Thus, her DL level is 20.
-Leo gains levels as if he is 2 levels lower than displayed. Thus, his DL level is 17.
-Corrin has Nobility, which boosts her Exp by 20%. Unless she is higher-levelled than the enemy (not too likely by the end, as enemy levels spike up quickly late), this works out to roughly gaining levels as if she were two levels lower, thus her DL level is 17.
Exp is heavilly curved in this game compared to previous FEs, and nobody joins too close to Endgame, so it's reasonable to take everyone else at the same level. Staff/dance exp is also very uncompetitive late, so no hyping that as a reason to take healers higher-levelled.
If someone wants to argue any of this feel free.
Equipment
Nothing breaks! But in Conquest at least, there's a reasonably tight budget. There's a total of about 125k gold in the maingame, and the party size is 16. Dividing one number by the other leaves about 7800 gold per PC, and we're probably burning 2k of that on a Seal. This number can be quibbled in various ways: you may miss some of these chests/rewards, but there's also more to be had from various optional Paralogues (which primarily depends on which kids you get and which ones you feel like doing). You may spend money on vairous non-DL stuff (healing items, tonics, staves/rods in general), you may splurge on core PCs while disadvantaging filler. etc.
Still, 6000 gold per PC (or perhaps 8000 - the number of seals used * 2000) is a pretty round number and I think it works pretty well as an equipment budget for each PC.
Forging
You mine gems of one type at a decent clip (2-4 per day), and can trade 5 of that type for 1 of another. There are six types in all, one for each weapon class. Once you have one of each, you can use the arena (refreshes every 6 hours or battle) to turn 1 gem into 2 or even 4 (or 8?) depending on your arena level. It's easy enough to get an ample gem supply for forging, the real limiter is money. To forge a +1 weapon, you need two of that weapon. To forge a +2 weapon, you need four. To forge a +3 weapon, you need eight. This rapidly gets expensive.
Legal weapons
In general, Bronze, Iron, Steel, and Silver can be bought in infinite numbers. Bronze boosts crit evade by 10, but can not crit, and costs 500. Iron is straightforward, and costs 1000. Steel reduces your effective speed for offensive doubling purposes by 3, and costs 2000. Silver is the best in the DL, despite being kinda iffy in-game: it costs 4000 and has tremendous power, but the downside is you lose 2 Str and Skl (Mag for tomes) every time you use one in combat, and this stacks.
An army of silver-wielders is not something I find terribly tempting in-game, but it will be the best option for many PCs, and I don't see any reason to find it illegal. It also rewards higher weapon ranks since most other weapons which do so aren't DL-legal.
Most other weapons can be bought in finite number. For instance, there is only one of each Brave weapon buyable, and 2 of each Killer. For the number of potential users, this doesn't strike me as legal. The exceptions:
-You can buy 10 each of Javelins and Hand Axes, which is more than enough for one each for any reasonable team, but may make forging them questionable. (Certainly +2 would be illegal). They cost 1800.
-You can buy two Levin Swords (cost 3300), range 1-2 swords which deal magic damage, and the only sword-user with high magic is Corrin. Thus, one is probably legal for her.
-Similarly, the Bolt Axe and Shining Bow also cost 3300 and deal magical 1-2 range damage for axe-users and bow-users, and you can buy two. It's hard to imagine more than two PCs wanting these (Camilla and MK Beruka miiight want the axe since Trample gives them decent enough Mag, for Niles and Shura it's the only 1-2 bow)
-Another bow option is the Mini Bow, again there are two, and two first-gen archers. It's a range-1 only physcial bow, and costs 2400.
-Nosferatu is back, nerfed but still useful in a duel. It costs 4000, and there are only two. By my views, there are three legal dark users: Odin, Nyx, and Leo... but you can argue it's unlikely you'll deploy all three as Sorcerers. Not sure.
I'm not sure if these will all be useful.
Even if you stick the PCs with the budget, the question becomes, what if they want to change? You could force them not to, you could let them change everything between fights, or you could have a default and make any changes only by the gold earned by selling things they don't need for the fight. I'm not sure myself.
Class choices
Tentative ideas I have, based on "one seal per character". This changes under different views, of course. Along with each class is the quick reason I went with each decision. Some may need a second look!
(spoilers, it's a PC list, don't read if you don't want to know it)
Corrin - Nohr Noble. Has access to Dragonstones, only unquestionably legal class, since you choose the second.
Felicia - Maid, only knife class. If not allowed Flame Shuriken, though, is probably better off with Strategist despite tome rank woes.
Jakob - Butler, only knife class
Elise - Strategist, tomes
Silas - Great Knight, better overall stats since move doesn't matter
Arthur - Berserker, speed + Axefaire
Effie - General, better stats and Wary Fighter
Mozu - Master of Arms, better caps
Odin - Sorcerer, Nosferatu + better mag/speed
Niles - Adventurer, Lucky Seven
Azura - Falcon Knight, best stats/caps. Songstress is unique but not DL-useful at all...
Nyx - Sorcerer, Nosferatu + better mag/speed
Camilla - Wyvern Lord after getting Trample, better stats/speed cap
Beruka - Malig Knight, Trample is probably worth the worse stats otherwise... unsure
Selena - Hero, Hand Axe access
Kaze - Master Ninja, better speed cap and Shurikenfaire
Laslow - Hero, Hand Axe access
Peri - Great Knight, better overall stats since move doesn't matter
Benny - General, better stats and Wary Fighter
Charlotte - Berserker, speed + Axefaire
Leo - Sorcerer, Nosferatu + best overall stats
Keaton - Wolfssegner, only legal option
Gunter - Totally unsure. He has access to Cav/Merc/Wyvern for branches. Malig Knight for Trample?
Xander - Great Knight for the same reason as the other Cavs, he's being wrecked by mages anyway
Shura - Bow Knight because it has better stats than Lucky Seven and he still has Lucky Seven
Flora - Maid, only knife class
Izana - Onmyoji, only tome class
These could use a second set of eyes to look 'em over.
I already have some average stat calcs mostly done, so I can post the early version soon if people are intereted. Still a lot more leg-work for the stat topic as a whole, though. There's also other new stuff to talk about, like debuffs.