As of now any new characters cannot be former X-Men, with the exception of Quicksilver and the Scarlet Witch. The characters we have right now are fine, but this is the Avengers, not the X-Men.
Also, no more of the Fantastic Four as PCs, due to future plot plans.
The Hulk is also an NPC for this game.
Here are some suggestions for PCs that might work well in the game:
Iron Woman (Tanya Stark)
Ant Man/Giant Man
The Scarlet Witch
Hercules (would be VERY interesting. If taken, I would advise you to contact Dark Deception and work out some backstory with his character Ares)
Luke Cage (Power Man)