Well, I'd say that Element Lad needs to be cast...and Karate Kid, for his dramatic entrance into the legion...Brainiac should be a challenge, though Michael Rosenbuam might be good there too...
And for some weird reason, I think that the late Jack Lord would make a good Lightning Lord...same names?
It just occurred to me that if they ever do make a Legion movie, it would be to Caucasians what Roots is to African-Americans. Man, that's a lot of pretty white kids.
As for Supergirl, well, it's a shame Judi Bowker (Andromeda from Clash of the Titans) is now too old to play the character, because she'd be my first choice. Since she is, hmmmm...how about Amanda Seyfried?
Element Lad = Colin Hanks (Orange County) - Element Lad needs an actor that is borderline between sensitive, shy and slightly on the nerdy side.
Shadow Lass = Laura Bertram (Andromeda)
Karate Kid = Adam Brody (the O.C., Mr. and Mrs. Smith)
R.J. Brande = Brian Cox (the Bourne Identity, Super Troopers)
As for Glorith - well, if they're willing to do color-blind casting, I'd love to see her as Aishwarya Rai; she'd make a pretty chilling villainess.