Beat the game to unlock these extra costumes.
Bobby Drake, Original Iceman
Colussus without metal, Colussus 90's Gambit's animated series outfit
Original Cyclops, Cyclops 80's
Original Jean Grey, Phoenix
Street Clothes Magma
Wolverine 90's, Weapon X
Finish Nuclear Facility level.
To unlock Cyclops Beat Mystique.
In X Mansion after Nuclear Facility level.
To unlock Gambit Finish Third Level.
To unlock Ice man finish the 1st level.
To unlock Jean Grey finish the 1st level.
To unlock Beast finish the 1st level.
To unlock Rogue finish the 1st level.
To unlock Storm finish the 1st level.
Finish Second Level.
Finish the second level.
Finish NYC Riot/Muir Island/Sewer level.
When going against Dark Blob, get as close as you can to him with storm, when he jumps at you/body slam, fly up in the air, if executed right, Storm will turn invisible.
First you need Collosus and Wolverine in your team, then when you do the throw move with Collosus, quicly tap B, A, and X at the same time, Wolverine should fly for 5-8 seconds and eventually fall back down.
To Unlock the Xtreme Costumes:
On the main menu enter UP, UP, RIGHT, LEFT, DOWN, DOWN, START
When you're on the Character Select Screen, Highlight a character and then press X to change into their Xtreme Costumes.