Marvel fighter: Professor X
Charles Xavier, AKA Professor X, is a powerful telepath, mutant, and the founder and mentor of both Xavier's School for Gifted Youngsters and a little group called the X-Men. He is considered the most powerful telepath on Earth, and a diplomatic and strategic genius who has led the X-Men for decades and even been one of the members of the Illuminati. As a telepath, his powers aren't exactly the best suited for combat, so a moveset for him would rely moreso on using illusions to hide himself and feign attacks, and also causing status effects, using psionic blasts to attack and shield himself, and even briefly controlling the opponent. His playstyle would center on mind games and faking out the opponent as much as it would on actually hurting them.
Aside from Onslaught, which is a fusion of him and Magneto, Xavier has never been playable in an MvC game, which I find strange considering how important he is to the X-Men and Marvel canon. His non-offensive powerset and pacifist personality, combined with being wheelchair bound, might have made it difficult to come up with a moveset, but I think it can be done and it's high time the Professor got a chance to show all these kids who the master is.
Capcom fighter: Edward Falcon
All these 2D fighters are neat and all, but what we really need is a character from a 3D fighter. Enter Falcon, adventurer extraordinaire, captain of the Hockenheim, and seeker of the mythical Power Stones. As the main character of Power Stone, he fits the stereotype of the all-rounder, but he has a few attributes that make him stand out. His fighting style is modeled after boxing rather than martial arts, and he can use moves like the Power Missile (literally firing missiles from his hands) and Power Hurricane (a rising upper that engulfs him in flame). Falcon could even take mechanics native to Power Stone with him; in Power Stone, the titular Stones spawn during fights, and if three are collected, the fighter will transform into a super form that grants access to more powerful moves. In Falcon's case, his Power Change is called the Red Whirlwind, where he dons what appears to be some kind of steampunk power armor (pictured below).
The case for Edward Falcon is simple: he's a cool guy who loves adventure and doesn't let anything get in the way (want proof? The only reason he even gets involved in the story of Power Stone 2 is because he
got bored and decided to head to an ominous flying castle to entertain himself). Other than that, Falcon would make for a welcome contrast with the four realistic Capcom fighters he'd be shown off with, and would assuage fears that the roster wouldn't have varied body types. And finally, Power Stone is getting remade soon so he would even be relevant depending on when this hypothetical game comes out! There's no downside to having him, so vote
Captain Edward Falcon!