Sinister is being saved for X4, where it'll be revealed that the Jean Grey in TLS was really just a defective clone (ala Madelyne Pryor) created by Sinister. The unstable clone teleported Cyclops to Sinister's hideout, where Jean has been in captivity, and Sinister is attempting to use Scott and Jean to create a mutant powerful enough to destroy Apocalypse. It all makes sense!
that was my idea back when X3 came out.
But you need to have the Fantastic Four find Jean Grey's body in stasis at the bottom of Alkali Lake in their new reboot movie (that would make a good post-credits scene