When it comes to the Marvel Cinematic Universe, the phrase "Never say never" is being stretched to its very limits, given recent surprises. Aside from Andrew Garfield setting precedence publicly denying any involvement in Spider-Man: No Way Home before his ultimate reveal, we can safely assume asking those "return" questions are pretty much useless now due to non-disclosure agreements. After all, Patrick Stewart declared himself retired from the role of Professor Charles Xavier from the 20th Century Fox era X-Men franchise with 2017's Logan before making his not-so-triumphant return in 2022's Doctor Strange in the Multiverse of Madness, only to be made short work of by Elizabeth Olsen's Scarlet Witch. Now with Hugh Jackman also unretiring as James Howlett, aka Logan, aka Wolverine, for Deadpool 3, it seems like Stewart might be stoking the fires of rumor for another return with his latest tweet and choice of words.
With the simple words, "Marvelous to be back in New York" next to the statue of Atlas, it's easy to be drowned in waves of speculation in hopes of seeing a full-on Logan reunion and finally, have Professor X prime appear with Ryan Reynolds' Deadpool. Despite not sharing any real scenes together, First Class Xavier James McAvoy did a cameo in 2018's Deadpool 2. While people can initially write off cameos by Stewart in MOM (not Earth 616) and Evan Peters (Quicksilver) in WandaVision, mutants in 616 officially became canon with the Disney+ series Ms. Marvel.
It kind of gets a bit complicated how X-Men can fit into 616 since we had world-type events that others around the world happen to ignore, or some galactic-level event could just make them appear in some plot device. Deadpool 2 was more than happy to screw with time travel, and Avengers: Endgame (2019) was more than happy to screw with its own timeline with the same concept. Knowing the MCU and Kevin Feige, it won't just be cheap fan service, and the appearances will be worthwhile.