2) Polaris
I mentioned the Scarlet Witch a little earlier, and speaking of Magneto’s children, Polaris is definitely a character with the potential to shake up these movies in a significant way. The green haired mutant’s story is an interesting one, especially as her relationship with her father is quite a bit more complex than the twins (though arguably a little too convoluted for a movie).
She does, however, share his powers, and if Michael Fassbender decides against returning in the next instalment (bear in mind that both his and the contracts of several other cast members came to an end with X-Men: Apocalypse), then bringing in a character with the same abilities could help fill that void. However, Lorna should by no means be left on the shelf even if the Master of Magnetism is back, because there’s plenty which could be done with her as a character in her own right.