Congratulations on adding Rogue, Gambit and Nightcrawler.
But I want to take this opportunity to ask the addition of the villains as well: Sabretooth and Omega Red.
It could also be created an X-Men tournament with the addition of these characters, something like Sinister Six.
I would like to see a 5* Mr Sinister in game. That would be terrific to say the least.
Toad and Blob!
Arch Angel
I’ve always see skills that increase other skill levels but not the other way around.
Omega Red’s tendrils can do that. Drains their strength (skill level) rather than health
Cloak and Dagger can actually do this already. It could easily be implemented in a different way (e.g. as an attack instead of a countdown) for Omega Red, which would be cool.
And then put them on the same team, and laugh as your opponent never fires a 2+ cover level power again.
I really doubt they’ll put that mechanic in more than once.
If they were both black abilities, it could be pretty hard to pull off in a reasonable time frame. Or there could be some other limiting factor. Have Omega only hit one Ability at a time, chosen at random, maybe?
Every iteration I’ve seen of him made him a total jobber but I’d love Sinister too. Give him passive bonuses if he’s teamed up with a Jean or Cyclops.
It would be funny if they release an Omega Red with a Tentacles ability that’s more useful than Doc Oc’s. Like how Rocket and Groot’s green is a better version of Banner’s transformation.
Sabretooth - yellow, red, and black colors. One power makes strikes. One power heals. One power makes strikes for both teams and damages the enemy and himself
omega red - red, yellow, purple colors. One power makes attack tiles. One power has sends tentacles that hurts specific players on enemy team. One power drains AP or health.
Emma Frost (Classic) - yellow, black, and blue colors, one powers steals AP, one power increases the amount of opposing AP needed to fire a power, one power creates strikes. Classic Emma doesn’t have diamond form.
Maybe Dark Beast would be possible? Not sure if he should count as a variant of 3* Beast or not, since they coexist in the comics.
Exodus would also be an interesting choice.
Another is Vulcan (the third Summers brother).
Jubilee! Can you imagine her firework powers destroying random tiles consitantly or changing tile colors! 
Some other suggestions:
Bastion/Nimrod
Shadow King
Mojo
Spiral
Silver Samurai
Lady Deathstrike
Dark Phoenix