Personally, I think it'd rock if Multiple Man was playable, but I just don't see that happening.
And if the game had Wolverine, Storm, Cable, and Deadpool, and that was it for X-characters, it really wouldn't feel like 'X-Men.' Give me Psylocke (unconfirmed but a possibility) and I'm 100% happy.
Though really, with Songbird already on the roster (I'm not saying they're similar, only their moveset and apperance might be), I'd rather have someone like Emma.
So Wolverine, Storm, Emma, and (Psylocke, Cable, Cyclops, Multiple Man) would be a good 4-X characters for me. I'm thinking Deadpool's gonna be in either way and I don't consider him an X-character anymore.
Well if they really want the best representatives of the X-men, they really need the heads of it. (Cyclops, Emma, Beast, Storm, Wolverine, Angel)
Personally as for "x-men" i don't think deadpool, cable, or bishop would sit well with me, neither would storm or wolverine for the fact there all over the MU now.
So for me, the X-men I feel are a must, that are not these. would be...
Cyclops, Emma, Beast and Angel would be the best representatives. Though I would also quite enjoy... (Colossus, Iceman, Nightcrawler, Gambit, Rogue, Xavier (in wheel chair), Psylocke, Jean Grey/Phoenix, Banshee, etc...)
I also wouldnt mind if we got 4 representatives of the younger X-men (Rockslide, Dust, Surge, and either Hellion, Mercury, Elixer, or Pixie)
random mutants I would personally enjoy as well (Multiple man, Marrow, Domino, Mystique, Toad, Avalanche, Quicksilver, Scarlet Witch, Havok, Polaris)