Quick resolve: introduce new characters in other books, bring them into the X-Books (but don't nececessarily make them mutants) afterwards. It'll be no different than if they added Hulk to an X-Team in the comics, FOX wont be able to touch him.
The X-Books only need new villains, so creating a non mutant villain in another series then bringing him/her over to the X-Books to be established as the big threat is definitely the way to go. Officially they'd simply be universally general (as in not confined to one franchise) villains; unofficially they'd be part of the X-Franchise (in comics; like Carol Danvers before 2006).
Gotta work around these things.