After watching DoFP, I started thinking about what new mutants could turn up in the sequel, so here is my take on it:
First of all Magneto, Mystique and Wolverine should only have minor or cameo roles, since the previous 5 movies revolved always around them, it's time to see some fresh faces. Most of their character threads have been explored but I am not against solo films for them, where they can shine in a different way. Magneto (Fassbender) I can see especially involving a backstory for his kids.
Apocalypse, 4 Horsemen: Quicksilver, Havok, Polaris, Psylocke.
They would be persuaded or partially mind controlled into service, as Quicksilver and Havok weren't exactly villain types in DoFP, although kinda shady figures (prisoner, thief). Polaris and Psylocke would be new characters in the movies and much needed female presence among the many male mutants.
There could also be several historical scenes, flashbacks, with previous incarnations of Horsemen, for example Sinister.
I would also like to see some villains recruited into the X-Men's folds -cue 4 Horsemen- as was commonplace in the comics, Banshee, Quicksilver, Rogue, Gambit, all started as villains, while in the movies it was always the other way around, everyone defected to Magneto in all of the movies.
Xavier, Beast, young Jean, Scott, Ororo and Kurt (all 4 recast obv.) around 18-20 years old students, plus Legion, Armor, Dust, Elixir, Proteus, 14-16-ish old students. This lineup would keep the gender balance and be more racially diverse as well. The younger students would have a distinct power-set, with mutations we have'nt seen in the movies yet, not just yet another teleporter, fire manipulator, etc.
I could see Legion (or Proteus) as the followup villain after Apocalypse, here only foreshadowing. Armor, Dust, Elixir, Proteus could be his personas at first, gaining separate bodies during the movie.
What do you guys think?