Right now I mostly just have a bunch of themes which I think would be fun to make into a game.
The most interesting thing about the X-Men setting to me is the school and the kids. The juxtaposition of the boarding school setting against the kids' fantastic powers and their mutant training is a great deal of the appeal. Mutant High is definitely inspired by comics and shows like New X-Men, The Runaways, or Young Justice.
Attributes would probably be things like Banter or Guts to support the genre. Emotional strength and will are more important than physical strength. If you want to be the Strong Guy then make a power to express that. Something like "That Rock Guy" or "Have I Told You About My Tactile Telekinesis".
On that note, powers are only important as much as they are an expression of the character. Spider-Man isn't interesting because of his powers, he's interesting because they empower him to express himself and to do the right thing. Pixie isn't interesting because she's a fairy girl who produces hallucinogenic powder, she's interesting because her mutations are a metaphor for her flightiness and her sweet disposition.
I'm thinking of using just 2d6 added to the attribute for action resolution. Powers are similar, 2d6+the power's rating. Characters should succeed at things which fall under their character's abilities (archetype?). However their actions should have consequences more often than not. You don't roll dice to succeed, you roll to decide what sort of consequence your action causes.