4) Emma Frost
Emma Frost did of course make a fairly sizeable appearance in X-Men: First Class, but that movie really didn’t do her justice. January Jones was criticized for a mostly wooden performance, and regardless of whether or not you agree with that, this version of Emma lacked the spark her comic book counterpart has. Rebooting her could be tough, but given her history in the Marvel Universe, it’s clear that she’s someone with plenty of stories left to tell.
Take for example the way she came between Scott Summers and Jean Grey; the divide created by the psychic mutant there would lead to her and Cyclops becoming something of a power couple when he took charge of the X-Men, while many members of the team doubting her new heroic nature was interesting to see play out, especially when it was confirmed that Emma had indeed embraced a more heroic path.
Even using her as a villain again would work to start with, especially seeing as the Hellfire Club played a role in the Dark Phoenix Saga.