Erik is so compelling, there's only a limited number of villains where the screenplay would be better than the Magneto story a decent Hollywood writer could pound out in their sleep.
Mr. Sinister can be Summers's archenemy. Mystique will always work, especially if both her children are in the film. The Inner Circle has legs, though you'd have to change things up a bit to give each one meaty hooks into a specific hero. You could do a good story where the X-Men instead of the Avengers save humanity from aliens.
A Soviet mutant with grappling whips, Arcade and Mesmero aren't on that list.