hmmm we do need more marvel girls than just storm... i wouldn't object to it.
come to think of it... these would be the most likely candidates considering they're marvel's top females. (excluding invisible woman and emma frost since they've been confirmed not in the game) and storm who is either confirmed or pretty much confirmed...
-phoenix (lots of energy projection, visually would be cool for the game)
-she hulk (would be fun, but we already have a hulk)
-scarlet witch (see phoenix)
-black cat (could be alright... but we already have a felicia

)
-elektra (possible, would play like a street fighter character)
-rogue (good chance since she's been in every marvel vs capcom game)
-ms. marvel (currently marvel's flagship female (so they're trying))
-black widow (hot off the heels of ironman 2)
other less popular choices but still possible would be...
-tigra (black cat would get the spot before her)
-dagger (i could see her pulled out just for an obscure part)
-medusa (see above)
-marrow (was in previous games)
-spiral (see above)
-lady deathstrike (i could really see her work well in the game)
-mystique (most likely not, her powers don't led any playability... just her fighting skills)
-X-23 (but we already have a wolverine)
so out of those choices... visually i think (phoenix, scarlet witch, ms. marvel, deathstrike and medusa ) would be most interesting and enjoyable playability wise.
but i wouldn't put it past them to include marrow or spiral or rogue again...