Awesome game so far, though Storm has my vote for the next character introduced. That would be totally hot!
Jean and Storm are the obvious big names to add, but IIRC Oni hates Jean though
![Frown :( :(](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
. Mystique (already added in the mod), Wanda, Amara, Jubilee, Tabitha, and Rahne are the other Evo girls from the cartoon that are possibilities. For other non-Evo characters, Psylocke is probably the most likely if I had to guess.
Someone who is unlikely to be added, but I would love to see is Magik (Illyana Rasputin). The younger sister of Colossus and one of Kitty's best friends in the comics. Her mutant powers are being able to teleport through time and space and she is also a high-level sorceress.
Most non-comic readers probably don't know her since she never was in any movies and cartoons (she is mentioned in Evolution), but she has become a popular character in the X-Men comics over the last 10-15 years since becoming a constant member of the X-teams.
Oni works at a snail's pace on this game so we are only likely to get a few more girls though. I feel the game needs to be fleshed out more storywise as well.