10 Unused X-Men Villains MCU Phase 4 Could Introduce
6. Fabian Cortez
Who? Fabian Cortez is a mutant who can actually enhance other mutants. And since this is not a list of heroes, you can gather that Cortez doesn't just act like a mutant version of the Vitamin-B shot. He can totally push mutant abilities too far and burn them out or make them lose control.
He's mostly known for being the leader/organizer of the villainous group "The Acolytes" who believe in the teachings of Magneto.
How he fits in the MCU: Again, I don't think the MCU X-Men should start out right away with Magneto as the big-bad. Involving Cortez and the Acolytes in the beginning gives Marvel a chance to tease his appearance and do some world-building (or is it altering at this point?).
If the MCU brings in the idea of mutants as a newer thing, rather than pretending they've always been in cannon, we could see a world where normal people are just starting to gain powers. Many of those people are going to be frightened of themselves, feeling persecuted, and will need someone to turn to for guidance...enter Fabian Cortez.