I seem to recall in the old forums we had a thread for wild, baseless speculation about how as-yet-unreleased characters could work, so I figured why not revive that sort of thread here? It was fun.
I feel like Quince should probably take a lot of design cues from Chizuru from KOF. Maybe Chizuru combined with Chris. He should have a fair number of moves where a second, illusory copy of him appears, clearly. I also feel like he should probably be one of the 5 HP Club, because of his presumable mixup-based gameplay.
A while back I summed up my feelings about what Menelker should be as "Divekick Grave." (This was back when there was debate about whether Grave should go from having a divekick to having a normal jump-kick.) He should definitely be a more aggressive, rushdown-based Grave with only 5 HP. Instead of wind I feel like an air fireball is guaranteed, but what to do with ground C? Maybe something like Valerie's air B? Like, a close-range crossup-oriented attack? I also feel like his Yomi counter should probably leave his opponent nearby, in easy continued-rushdown range. And obviously his super should be the Deathstrike Dragon for 3 damage. (I also think his stance should be Grave's except with his back to the camera, and slowed down considerably)
Some of the other Shadows characters seem like they would be really challenging to adapt to an actual fighting game. Persephone might work as a high-mobility grappler kind of akin to R. Mika or Shermie? Gloria seems really difficult to put into a fighting game without breaking the game. Gwen's shadow plague still seems like a problem without a convincing solution (give her Mech Zangief armor? Keep her at one-hit-chip status at best? Give her — gasp — only 4 HP?).
Some characters seem simple, though. Troq should have 7 HP and maybe play like a combination of T. Hawk and Boxer, I think. Lots of charging and an anti-air, and probably 2-damage throws? Actually, this is less straightforward than I thought.
Onimaru could be interesting. Not really sure how to best adapt his "single big hits" gameplay from Yomi, if at all. Grave has that sword move, which seems like it would be perfect for Onimaru. Hmm.
Another interesting question is whether we might see any of the new heroes from Codex in here, even in theory. Characters like Bigby and Drakk have zero fighting game analog at all, essentially by definition, so they're more or less blank slates should things eventually make it that far in the year 20XX.