My account is pretty old but I just recently started seriously playing, here's a list of heroes I was surprised aren't on the roster.
1 Jubilee. She was a major character on the 90's animated series and noticeably missing from the roster.
2 Mystique. This is a no brainer, she's a major presence in the comics, animated shows and all the new movies, how she's not in the game is mind boggling to me.
3 Jessica Jones as a playable character. Hit show on Netflix. Do I really need to get into detail on why she should have already been playable? You're basically leaving money on the table Gaz.
4 Bishop. I know I know, he's not as "important" as the previous 3 but Bishop made a big impact when he 1st appeared alongside Cable. He's also one of the handful of black male heroes, and you know, diversity's always a good thing.