Assuming it'll still be in the 60's or 70's, I'd love a teenage/in his 20s Colossus defecting from the USSR and enrolling in Xavier's school. I think it'd be great, not only would he be a mutant (With a really cool visual, by the way), but there'd be the added paranoid of him being a Russian at the height of the cold war. I know he showed up in 2, but only the nerds knew who he was.
Sunfire: His origin is tailor-made for this universe. This guy developed his powers while America dropped the A-Bombs on Japan. His regular costume is pretty terrible, but google his Age of Apocalypse one, it's a great design.
Apocalypse and Mister Sinister: At least sometime down the road. It'd give Magneto's new Brotherhood a reason to team up with Xavier's school as the world's oldest mutant with a Darwinian agenda shows up. Also, the character has a very cool appearance. Mr. Sinister is just a great villain, an insane geneticist obsessed with mutants.