I'm procrastinating going to work right now so thought I'd leave this here for fun.
Bishop - Could be cool. I always liked him, but he's kinda just Cable with energy absorption powers so maybe not.
Wonderman - This, I think, would be awesome. His ult could be something like where he goes full ion form and just nukes the crap out of everything.
Sentry - I think this would be pretty sweet. There isn't really a "Superman" type in the game right now that I can think of and the possibilities for builds would be pretty vast. Ult could be a "lose your mind and go full void and wreck the planet" lol
Professor X - This would just be cool. I mean he is one of if not the most powerful mutants on the planet. Plus can you imagine a movement build with this guy? "I'LL RUN YOU OVAH WITH MAH HOVAH CHAIR!"
Thanos - I know the story is looking at bringing Thanos in anyway, and personally, he's my favorite villain in Marvel. Possibilities for powers/builds here are crazy as well I think. He brings a lot of versatility in his power set and I think his playstyle could be pretty unique if done right.
Nate Grey - He's Nate Grey and he's awesome. Nuff said.
Odin - How cool would it be to beat people down as the All Father himself? Probably never happen in a million years but still.
Morbius - Blade needs someone to hunt right?
Gladiator - Could be fun. Maybe a confidence resource?
Kurse - Has the potential to be a really fun brawler character.
Molecule Man - Just because I'd be really curious how they'd do his powers. Guy can basically do anything.
Quicksilver - Because we need a speedster in the game IMO.