Doctor Strange in the Multiverse of Madness almost featured an appearance by Cable and Deadpool -- emphasis on "appearance." The duo would have shown up on a billboard reminiscent of the Deadpool 2 marketing campaign, but not actually shown up in the flesh (and metal) during America Chavez's uncontrolled descent through the multiverse. The Marvel Studios: Assembled episode for Doctor Strange in the Multiverse of Madness revealed as much, in addition to giving fans a look at concept art from the scene, which was likely never filmed.
The art features two different images (one repeated): Deadpool and Cable recreating Da Vinci's "The Creation of Adam" painting, which was used in the advertising for Deadpool 2, and one of Deadpool in an Uncle Sam "I want you" pose and hat.
You can see it here:
The film did, of course, feature a look at Charles Xavier (Patrick Stewart), transplanted from Fox's X-Men movies into the MCU (albeit on another Earth, where the casting may or may not match up with what we eventually get). There was no real context for his role on that world, other than what we can assume from him being Professor X.
You can check out the official synopsis for Doctor Strange in the Multiverse of Madness below.
Now that Iron Man and Captain America have left after a fierce battle in Avengers: Endgame, former genius surgeon and the strongest magician of them all, Doctor Strange, is expected to play an active role as a central figure in the Avengers. However, using his magic to manipulate time and space at will with a forbidden spell that is considered the most dangerous has opened the door to a mysterious madness called "the Multiverse."
To restore a world where everything is changing, Strange seeks help from his ally Wong, the Sorcerer Supreme, and the Avengers' most powerful Scarlet Witch, Wanda. But a terrible threat looms over humanity and the entire universe that no longer can be done by their power alone. Even more surprising, the greatest threat in the universe looks exactly like Doctor Strange.
Doctor Strange in the Multiverse of Madness is currently available to buy or rent on digital video platforms like iTunes and Vudu.