Magneto actor Michael Fassbender says it's possible he returns to the X-Men franchise for the upcoming installment X-Men: Dark Phoenix. It was a risky proposition for all involved when, back in 2011, the X-Men franchise was softly rebooted with the prequel film X-Men: First Class. Among the big changes made for the prequel was the recasting of most of the series' main characters, including Charles Xavier aka Professor X and Erik Lehnsherr aka Magneto. As it turned out, audiences took to the new incarnations of the X-Men, and there have been two more X-Men films with the "new" cast headlined by Michael Fassbender as Magneto, James McAvoy as Professor X and Jennifer Lawrence as Mystique.
For the next entry in the X-Men franchise, Fox is planning to tackle the fan-favorite comic book storyline "Dark Phoenix Saga," which was first attempted to largely negative fan response in the 2006 movie X-Men: The Last Stand (which was in fact the last stand for the series' original cast except for Hugh Jackman). Sophie Turner is reported to be involved in X-Men: Dark Phoenix (originally entitled Supernova) as Jean Grey (who of course becomes the Dark Phoenix) but beyond that casting news has thus far been sparse.
One X-Men: Dark Phoenix tidbit that has been reported is the possible involvement of Michael Fassbender, who is currently busy promoting his new cinematic endeavor Alien: Covenant, in which he portrays dual android roles as Walter and David (the latter of which appeared in Prometheus). Fassbender addressed the reports of his returning to play Magneto on an MTV podcast hosted by Josh Horowitz and indicated that he will "likely" be on-board for the Dark Phoenix film. His possible involvement in New Mutants remains up in the air (James McAvoy already is reported not to be appearing).
In the Dark Phoenix comic book arc, Jean Grey is exposed to solar radiation which enhances her already-existing telepathic and telekinetic powers to the point where she becomes a being of pure thought. Grey tries to bring her own massive powers in check but after a failed battle with Magneto she loses control of herself and spirals into becoming Dark Phoenix with a little help from Mastermind and Emma Frost.
Cyclops and Professor X are also central characters in the Dark Phoenix story, which could mean big roles in the new film for Tye Sheridan, who played Cyclops in X-Men: Apocalypse, and James McAvoy. After some audition tapes surfaced on the web, there is also a rumor that Rogue will return for Dark Phoenix with a different actress than Anna Paquin, who notoriously was cut out of X-Men: Days of Future Past (though a so-called "Rogue Cut" of the film with Paquin back in ultimately came out).
Outside of the casting, it will be interesting to see how Marvel and 20th Century Fox approach X-Men: Dark Phoenix. After the success of the R-rated Logan, there may be some inclination to go darker and gritter in future X-Men films. On the other hand, the X-Men films up until now have succeeded with a well-established formula that keeps the violence on a strictly comic book level and relies more heavily on time-tripping acrobatics and spectacle, so perhaps going full-James Mangold wouldn't be the best approach.