There are certain characters people keep listing that are, quite frankly, NEVER going to happen:
Hank Pym/Wasp; I just don't see how they could make the size changing work in this kind of multi-player enviroment.
Cable; A character that seldom appears outside X-titles, whose powers are mostly overshadowed by his stupid, giant, future guns. Until someone finds a costume for this guy that doesn't suck, and makes him something other than an angsty '90s goon with powers he can barely use, not gonna happen.
Psylocke; I like Psylocke, but just like Cable, she's a relativly obscure X-character, and her powers in XML weren't great.
Bishop; See Cable.
Shang-Chi; Redundant Iron Fist. If he makes it at all, it will be as one of Danny's costumes.
Punisher/Nick fury; One or the other, both becomes redundant.
Shadowcat; Intangability was used resonabley well for Martian Manhunter in Justice League Heroes, but they'll want to prioritize that power for Vision, so Kitty would probably only be playable in X-men games.