'Doctor Strange in the Multiverse of Madness' could introduce the Illuminati and set up 'Secret Wars' as the next major MCU crossover
The concept of the Multiverse was introduced to the Marvel Cinematic Universe (MCU) in the very first 'Doctor Strange' movie. Judging from the title, the sequel film 'Doctor Strange in the Multiverse of Madness' will be further exploring the concept and there's a particular storyline from the comics that seems like it might be ideal for the movie.
2015's 'New Avengers Vol 3' introduced the universe-shattering phenomenon known as the Incursions, which eventually led to the massive 'Secret Wars' crossover. The Incursions were cataclysmic events in the comics where one universe bleeds into another, destroying both.
The only way to survive an Incursion is to destroy the other Earth before the situation becomes critical, creating a very difficult choice for our heroes to make: doom a whole other universe, or die. Doctor Strange may not be a medical surgeon anymore but he is still unlikely to go against his oath to help protect all life, which means this choice would weigh heavily on the Sorcerer Supreme.
If he is to protect the Earth from the Incursions, he's going to need all the help he can get, which brings us to the Illuminati. In the comics, the Illuminati (no connection to the secret society) was formed by a group of the brightest minds on the planet including Tony Stark/Iron Man, Reed Richards/Mr. Fantastic and Doctor Strange in order to work behind the scenes and ensure the safety of the Earth.
After the planet was first threatened by an Incursion, a new version of the group was put together by T'Challa/Black Panther (Chadwick Boseman) with Steve Rogers/Captain America, Mr. Fantastic, the Inhuman King Black Bolt, Namor the Sub-Mariner, Doctor Strange and Tony Stark.
There have already been quite a few teases and rumors that have suggested Marvel may be looking at the 'Secret Wars' storyline to be the next big MCU crossover so it makes sense that Doctor Strange 2 might introduce the Incursions and the Illuminati. 'Secret Wars' has happened twice in Marvel Comics, once as a limited series in the 1980s and a second time in 2015, with both of those events bringing pretty much every popular hero and villain together for a massive crossover.
It's also quite easy to see how the Incursions may be seen as a "madness" gripping the Multiverse and the Sorcerer Supreme may find that he needs to put together a team of pragmatic heavy-hitters to deal with this insanity. With Robert Downey Jr.'s Tony Stark dead and Chris Evans' Steve Rogers happily retired, we will probably be seeing a whole new iteration of the team, if and when it forms in the MCU.
Black Bolt is technically a part of the MCU already and both Namor and Reed Richards are expected to make their debuts in the shared universe soon. In any case, an event of such cosmic proportions is unlikely to be wrapped up in just one movie so we may see the Incursions pop up in multiple films, culminating in a 'Secret Wars' movie sometime in Phase 5.
'Doctor Strange in the Multiverse of Madness' will arrive in theaters May 7, 2021.