Marvel fans are already theorizing how Patrick Stewart, Ian McKellen, and James Marsden's X-Men will be integrated into Avengers: Doomsday

Following Marvel’s huge Avengers: Doomsday cast announcement, which includes a group of Fox X-Men stars, fans are already wondering how the X-Men will fit into the MCU movie.

Marvel fans have been rallying around a Reddit thread to discuss the news and their theories, with many thinking that the X-Men being involved in an Avengers movie could only mean one thing: a call back to the 2012 Avengers vs. X-Men comic. "Is this gonna be X-Men vs Avengers? Cause the cast is way too big for them to make any sense of it any other way, right?" said one Reddit user.

At this time, there are four original X-Men stars set to come back, including Patrick Stewart as Professor X, Ian McKellen as Magneto, James Marsden as Cyclops, Channing Tatum as Gambit, and Rebecca Romijn as Mystique. However, The Hollywood Reporter has confirmed that there are more casting announcements on the way, which could mean more X-Men, maybe enough for an Avengers vs. X-Men side storyline?

However, other fans think that Avengers: Doomsday will involve a timeline incursion that will merge the Fox X-Men timeline with the current main Earth 616 timeline, just like Deadpool and Wolverine did. "There's multiple incursions going on," pointed out another fan. "There's definitely the one that Dr Strange caused."

One fan thinks that the X-Men will only appear in "a scene of their universe getting destroyed," showing the Avengers what villain Doctor Doom is capable of. "I mean it’s the only way a new era of x men can be born," continued the fan. "Realistically, Ian and Patrick are mid-80s. They can’t be pivotal characters anymore, as legendary as they are."

Although Doomsday will mark McKellen, Marsden, and Romijn's MCU debut. Stewart has already appeared in the Extended Cinematic Universe in Doctor Strange in the Multiverse of Madness, where we saw Professor X die. This means that the Charles Xavier in Doomsday won’t be the same one as before; perhaps he will be from the same universe as Deadpool 3’s Wolverine variant.

Avengers: Doomsday hits theaters on May 1, 2026. For more, check out our guide on how to watch the Marvel movies in order, or keep up with upcoming Marvel movies.