The contracts of all the X-Men franchise's stars ended with X-Men: Apocalypse earlier this year, and while some (James McAvoy) are more keen to return than others (Jennifer Lawrence), we simply don't know what's next for these characters. There's been talk of a soft reboot and a sequel focusing on the younger cast members, but nothing is set in stone, especially after the latest release underperformed.
During the recent press tour for Assassin's Creed, Michael Fassbender was asked about possibly reprising the role of Magneto, and it doesn't sound like Fox has reached out to him as of yet. "I don’t know to be honest," he admitted. "I haven’t made any sort of decision on it because I have no idea what’s happening next with the franchise to be honest." That doesn't sound good, but you can't blame him for taking a wait and see approach before signing on to anything else with the studio.
Still, with Assassin's Creed not doing well critically or financially (so far), Fassbender could be glad to return to X-Men as Magneto if that chance arises. However, that's a character we've seen a lot of over the years though, and there's a chance he'll be shelved for the spotlight to be moved elsewhere now. Time will tell, but be sure to let us know your thoughts on the actor's comments in the usual place.