Andy has given us statheads the task of figuring out the generals of skill use; How skills are learned, their general usages, and enumerating in general terms what resources characters would be using. Basically, "who has what, how they get that what, and what it costs" --General terms, not overly specific, mind you.
As such, I'll try my own attempt on the six or so that are notably pinned down; Katarine and The Person Formerly Named Faulheit are going to be excepted for now because nobody seems to be able to agree on the former (especially now that we're reaching critical mass for pure casters), and the latter's undergoing a retool anyway.
Noemi--Variety of magic in general. (Offense, status, buffing, maybe a token heal later on)--she's a magical jack of all trades. Oh, and Prodigy to give her any other spell she may want from an ally for the fight. Depending, she may have a physical technique or two to use as well, but she's more variety than focus; master of none.
Prodigy should be started with, and as for the rest...well, she might be a decent starting point for basing how magic is learned in general. My kneejerk would be a skillpoint system (scripts exist for both engines), but that would then make me want to force a branching setup for spells--and I know Andy has stated he wants innate character progression to be linear, not branching, so I'm lacking in ideas for the time being.
I say she's a candidate for being of the multi-element persuasion, regardless; fire (Courtesy of Erastus' tutelage) and one or two others.
Mirek--Mirek is weird in that he's being expanded from his old self, rather than contracted, gameplaywise. But as such...largely varying physical attacks, and support abilities that are focused around defending against magic--both personally and for the party.
Largely, Mirek would probably be more TP-oriented than most--with the ability to smash the ever-living crap out of someone should his TP get maxed. (Note to self: there should not be a way to get a quick TP boost). Might either use MP or cooldowns for his more commonly-usable abilities.
Slashing-only as far as elements go. Yeah, he's that limited--unless we're going with sheathed attacks, in which case most of his attacks would be Impact.
Isolde--Isolde's largely being expanded in a different way--and reined in from her old incarnation. I guess I'll give in and go with the ice/earth everyone's pushing on her. :/
That aside, seeing Isolde as a mix of Magic, Physical, and Composite damage skills here--practically everything she has does damage of some sort, and if it doesn't have a rider effect (which would usually amount to debuffing enemy defensive/evasion stats in some capacity), it's often busy doing -more- damage. Oh, and Oversurge lets her get a shot of extra damage at a cost.
As far as resources go? MP at the very least, with Oversurge being a skill that costs HP straight up for an instant one-shot boost to an attack (but also increases MP costs for that attack to prevent abuse). May also factor in TP or cooldowns pending on how we're doing physical and composite skills on average.
Kasia--We'll be honest. Kasia's a healer. That means she's going to have the standard healer options (healing, possible status heal, revive) at some point--though with these all inflicting some sort of negative status on the recipient in turn. That aside, she's going to have her selection of status options, both through magic and through archery, the way I see it--we need fewer pure casters anyway. Standard status fare, I guess (Poison, blind, etc), with archery stuff doing damage as well.
Resources? ...yeah, MP for magic, TP/cooldown for physicals, assuming MP isn't going to be a general-use SP? I admit I'm slightly in favor of TP being used as more of a WA FP style thing here for most characters, but at the same time it's due to me liking to see basic physical use kept to a minimum--but that's my bias.
Erastus--Nothing really deep. He's a farking fire nuker mage. Could give him some sort of TP skills to expand upon his skillset, but as far as I'm convinced he's going to be largely one-dimensional. He's artillery, he'll blow stuff up--see later on how I'm seeing offensive elemental magic working.
Eirwen--Okay, so she's going to be a fast attacker, and the game is apparently going to have two entirely different classes of staves. Still say there's D3 influence here.
That aside, I admit I'm not keen on the notion of making her absolutely dependent on weapon switching. However, I have a better idea on her resources than any of the other characters here, oddly enough: Rapidly regenerating TP, with most of her skills running off of that (and some usable turn 1, even). Cooldowns on a few of her skills (particularly First Aid and Resuscitate, her skill-based healing), and then she has Hi-Item access. Keep in mind, Hi-Item can encompass more than the better healing items, and if we are going magitech, there'll likely be bombs or the like in there somewhere.
Element quirks in general: I will admit, if it's one thing I utterly and completely detest, it's copypasta elemental spells with nothing differentiating them outside of element. It's boring, unimaginative, and needs to go, so I'll try to address this now. Properties for generic elemental spells, assuming I'm getting the list for proper offensive magic elements right (If not, poke me and I can include the others) (Unique stuff can break with these properties as much as it wants, it's unique and all that crap.):
Fire--Tends toward multihitting at lower power. In turn, it's generally sensitive to MDEF, and even merely above-average MDEF can put a damper on it. In short, it's Faulheit's wind spells from the old IAQ
Ice--Ice is more on the slightly low-powered side, but with a tendency to delay the opponent it hits.
Lightning--Super-accurate, but unreliable when it comes to its effects; very high damage variance means it can hit hard, or barely touch you at all. May also have a notable critrate for additional gamble-time bullshit. MT variants may trade this in for being rando-target off of shoddy accuracy instead.i
Earth--Earth is arguably the "power" element here, the way I'm proposing--not MDEF sensitive and good power with particularly low variance. But it's expensive, and has lower accuracy compared to the others.