The new mutants- Cannonball, Sunspot, Moonstar, Magma, Karma, Wolfsbane. I would also add Warpath, Frenzy, Chamber, Husk, Sage. From the new xmen generation- Hellion, Surge, X23, Dust, Hope Summers, Tempus, Hijack
Magneto definitely. After that who knows... the gap is huge. I guess you could throw a bone to Cyclops and Storm. Really there hasnt been much in the way of character development or even great stories excpt for with Magneto
Hmmmm.... as far as skill, I would say I guess Xavier, Jean, Emma and believe it or not Quire (as much as I hate him). As far as the villains go... Cassandra Nova (still not sure if shes alien or whatever), Exodus (I think he's a telepath and telekinetic), Maddie Pryor, Stryfe, Sinister.
An X-23 movie, if done right could be a big deal. Especially considering the current push in the ya genre of books turned into movies coupled with the superhero and clandestine special ops movie market that exists. My pick to play X-23 is India Eisley all the way.