Marvel
Beast: He's always been my favourite Marvel character, Acrobatic beat downs would be fun, though you could say Felicia has that kind of covered already minus the extra impact I assume Beast would bring.
Kingpin: I have no real idea why this came into my mind, I guess the idea of punching someone across the screen as Kingpin amuses me or something.
Gambit: Just because my friend would pay any price for his inclusion
Capcom
Sissel: I'm not sure how it would work but object possession and being a ghost could be quirky and fun.
MegaMan Classic: Not an exciting choice but one that really should be in the game anyway in some form at least.
Agrippa: Remember Shadow Of Rome? no? well I guess he'd play like Arthur but with a variety of close to mid range weapons instead, oh and the German suplex of course. This doesn't fall under the history problem right?