With Marvel's introduction of parallel universes, and the concept almost certainly followed in Spider-Man 3 and Doctor Strange 2, it seems likely that any character from any previous Marvel movie could re-emerge.
Rumor has it that in the next phase, we may soon see three generations of Spider-Man, Blade Warriors, characters from Netflix Marvel episodes, and other characters, among others.
And the latest news is that the old X-Men character will appear in the Marvel Cinematic Universe, and Patrick Stewart may play Professor X again.

The news comes from Wegotthiscovered, which says Marvel is in talks with Patrick Stewart to play Professor X again in Doctor Strange 2: Crazy Multiverse.
It's no secret that Stewart has already discussed the return with Kevin Fitch. Just a few weeks ago, Stewart even said that playing Wolverine 3 changed his mind about returning to the role.
For now, it seems likely that Feige persuaded the old "Professor X".
Patrick Stewart has played Professor X in seven different X-Men movies — starting with X-Men in 2000 and ending in Wolverine 3 in 2017.
For both Patrick Stewart and Hugh Jackman, Wolverine 3 seems like a farewell to the X-Men characters — especially considering that both characters die at the end of the film. But considering that the "Restructured History" design has already appeared in the Avengers and X-Men movies, it seems that Feige has already figured out how to reintroduce the two (does that mean... )
Since Stewart left the role, Professor X has been played by the popular "One Beauty" James McAvoy. Although Stewart may make a cameo appearance in the role again, considering the popularity of Ichimi, it may be the highlight if Ichimi appears in a Marvel movie.
However, as early as 2019, Kevin Feige said that it will be a long time before X-Men joins the Marvel Cinematic Universe. Marvel seems to have reservations about the X-Men, instead focusing on reintroducing other characters. Mahersala Ali's Blade Warrior is currently filming its fifth stage, while Spider-Man director Jon Watts has signed to direct The Fantastic Four.
But Marvel has been tight-lipped about how to introduce the X-Men in the Marvel Cinematic Universe. Either the casting is far away, or they haven't even started deciding who will star in their new versions of Professor X and Magneto, and so on.
Will Professor X reappear on screen? Is it possible for the new and old Professor X to be in the same frame? Let's wait and see.