The X-Men may be well on their way into the Marvel Cinematic Universe, but not as they've been seen before. This means that one X-Men star might see his return to Marvel, but not as a mutant.
The X-Men soft reboot, starting with X-Men: First Class (2011) and concluding with X-Men: Dark Phoenix (2019) gave a more youthful and refreshed look at the beginning of the X-Men, where the actor Michael Fassbender was known popularly for his role as Erik "Magneto" Lehnsherr. After Marvel Studios earned the rights to the X-Men following 20th Century Fox's universe, Marvel president Kevin Feige announced that the X-Men were to return with a new cast, meaning that Fassbender's time as Magneto had come to its end.
It has been rumored that Fassbender is set to re-enter the Marvel Universe in some capacity after his departure from the X-Men series, and it is heavily rumored that he is currently being scouted to play Dr. Victor Von Doom in the upcoming rehash of the Fantastic Four. The announcement of a new Fantastic Four project is fairly fresh, but the bruit around Marvel seeking out Fassbender for the big-bad role indicates that it's a studio priority.
Spider-Man director Jon Watts is apparently poised to direct the movie focused around Marvel's First Family, and with Marvel looking to significantly alter their brand through new projects exploring new characters, stories, and potential cross-overs, it could mean that Fassbender's Dr. Doom could become the next big foe. Neither Marvel nor Fassbender have commented.