Patrick Stewart's Charles Xavier. That particular film promises to transform the entire MCU in a multiversal catastrophe that tests Doctor Strange and the Masters of the Mystic Arts to their limits.
Marvel Studios president Kevin Feige has long hinted he expects Doctor Strange 2 to redefine the MCU, setting up major supernatural elements for the MCU's future - and potentially more besides. What's more, recent movies have suggested there could be changes on an unprecedented scale; Avengers: Endgame, Marvel Studios clearly don't mind differentiating it from the real world anymore.
This is particularly important because Doctor Strange 2 could draw ideas from the resolution of the comic books' event. This was a multiversal catastrophe at an unprecedented scale, and in the end the super-powerful Franklin Richards used his powers to restore reality. It didn't quite go right, though, because fragments of other dimensions such as Weirdworld were left stranded on Earth. Should Marvel take a similar approach in Doctor Strange in the Multiverse of Madness, they could incorporate fragments of any dimension Doctor Strange visits in the film into the MCU's Earth. That includes a prehistoric dimension briefly glimpsed in the second Doctor Strange 2 trailer - which could become the basis for the MCU's Savage Land.
Like Madripoor - which appeared in Savage Land is closely associated with the X-Men. Created by Joe Simon, Jack Kirby, and Syd Shores, this mysterious location was integrated into the modern Marvel Comics universe in X-Men #10 when Professor X believed he'd detected a mutant there. The comic book version of the Savage Land is a prehistoric land hidden in the depths of Antarctica, protected for millennia by ancient aliens; it's the center of a network of portals that crisscross the globe, and as a result is inhabited with countless different tribes. Fans have long been eager for the Savage Land to become part of the MCU, and Doctor Strange in the Multiverse of Madness could finally make that a reality.
Of course, if this theory is correct, the MCU's version of the Savage Land would have to be a little different. Exploiting the MCU's multiverse, it would need to start out as a dimension where dinosaurs never died out - perhaps where an asteroid never struck the Earth, triggering an extinction-level event. A fragment of this reality would then need to be left on Earth in Antarctica, perhaps protected by the magic of the Masters of the Mystic Arts because they considered themselves somehow responsible for the chaos that has unfolded. That rewrite seems quite easy to implement - meaning it really is quite possible Doctor Strange in the Multiverse of Madness will introduce this prominent X-Men location into the MCU.
