I'd love to see an X-4. I really refuse to believe that the X-Men's story ends there, on film at least. Bring back Jackman as Logan, and more flashbacks, ala X2 (depending on how it's handled in X-MO:W), with Marsden stepping up and leading the team as Cyclops, along with Stewart as Xavier. Gimme a modern day version of Days Of Future Past with the humans finally going crazy on mutants and winning, along with Sentinels, and you've got my ass in the seat at the first midnight screening.
Other than that, I'd like to see more spin-offs, like a Wolverine one that maybe takes place after X-3, depending on how X-MO:W again and maybe Deadpool.