7) Vulcan
In Deadly Genesis, it was revealed that Professor X had assembled a team of X-Men who died during their first mission. One of these mutants was Gabriel Summers, the secret brother of Scott and Alex. He was kidnapped as a baby and brought up by the alien race, the Shi’ar. After returning to Earth and being recruited by Charles Xavier, Gabriel was left in a form of hibernation for years before waking up with an overwhelming desire for revenge for that ill-fated mission.
This storyline led to Cyclops kicking Charles out of the X-Men (resulting in him becoming their leader), while Vulcan would return to outer space to attempt to conquer the aliens who stole his childhood. It goes without saying then that his inclusion in the X-Men: Apocalypse sequel would open the door to many new storytelling possibilities and could very well take this franchise in a cosmic direction, which will help to freshen things up.