Completely random thought - I feel like a Shin Megami Tensei fighting game would be really cool... on the condition that it deliberately makes a point of not just focusing on all of the protagonists. I mean, SMT is pretty much defined by its side characters and demons to the point where a lot of the protags don't even have their own names, so, y'know... that aside, a decent mix between them would be a lot of fun - imagining potential character candidates like Mothman, Heat, Shiki-Ouji, Matador, Terminator and so forth could be fun as hell.
Come to think of it, I suppose you could even seperate characters into different functional archetypes that indicate basic elements of their playstyle. Spitballing here, but maybe a little something like Demon, Summoner and Persona as well as a couple of unique ones like Demonica or Atma for specific, singular characters. Seems like it'd be neat!
I suppose the other way of doing it is to have you select a (or multiple) demon summons with their own unique abilities and then fight with a character capable of summoning them (such as characters with COMPs or Wild Cards), though I feel that might be a nightmare to balance out, and could run the risk of making some characters difficult to implement (such as non-protags/Velvet Room attendants from the Persona series).