This means, likely, that characters from older Marvel movies are now ripe for incorporation into the MCU. This is, of course, a natural extension of some of the other cameos already provided in previous movies. Tobey Maguire, Andrew Garfield, and Tom Holland all appeared on screen as parallel Spider-Men in “No Way Home,” and “Multiverse of Madness” already featured Patrick Stewart playing Professor X from the “X-Men” movies.
In the upcoming film “Deadpool 3,” Hugh Jackman will similarly return as Wolverine, although which Wolverine from which universe remains unclear. But we might have gotten a hint, thanks to the mid-credits scene in “The Marvels,” where Monica Rambeau ends up in an alternate universe. This scene not only includes an appearance by Kelsey Grammer as Beast from Fox’s “X-Men” movies, albeit with a CGI makeover, but there’s also a different version of Monica’s mother Maria Rambeau (Lashana Lynch) who is alive and well and has become the hero known as Binary in this universe.
The book points out, however, that there is a Sacred Timeline (something talked about in “Loki”) that doesn’t include every single Marvel character from throughout the media’s history.
Confusingly, in “Multiverse of Madness,” the “main” universe of the MCU was referred to as Earth-616, the name most often given to the “main” universe in Marvel Comics. It seems that the MCU is trying to stand in as the live-action version of Marvel Comics. The MCU does not exist in a world parallel to the printed page.
But everything may be fair game. Feige wrote in his foreword:
“On the Multiverse note, we recognize that there are stories — movies and series — that are canonical to Marvel but were created by different storytellers during different periods of Marvel’s history. The timeline presented in this book is specific to the MCUs Sacred Timeline through Phase 4. But, as we move forward and dive deeper into the Multiverse Saga, you never know when timelines may just crash or converge (hint, hint/spoiler alert).”
The “Hint, hint” and “spoiler alert” could merely indicate that Feige is being playful, although it’s just as likely he is referring to an upcoming multidimensional conflagration that would compress multiple timelines into one.