1) Fantomex
Fantomex is a character with a VERY complicated origin story, and it would be essential to simplify that if he were to be brought to the big screen. A creation of the same people who gave Wolverine his Adamantium skeleton, Fantomex could easily be added to pretty much any plot, and he’s one of only a few X-Men who’s capable of carrying his own movie.
He’s also been a member of recent versions of X-Force, so if that spinoff is still happening, an introduction here for Fantomex would be ideal. Highly cunning, skilled in misdirection, and a master thief, this guy even has a ship which is an extension of his nervous system named E.V.A. Needless to say, he would be an exciting and original addition to the next instalment in the what many call a now stale franchise.