Marvel Studios secretly set up Professor X (Patrick Stewart), and other unidentified . It's a moment that caught many by surprise, even though Marvel planted the seeds for the team's multiversal formation years ago.
The scope and direction of Doctor Strange in the Multiverse of Madness seemingly changed multiple times throughout the course of its development. The original Doctor Strange ended with the arrogant sorcerer learning about Infinity Stones and his role in protecting Earth from supernatural threats. However, the post-credits also featured Baron Mordo beginning a quest to rid the world of sorcerers who abused their power. This led many to think Baron Mordo would be the main villain of a Doctor Strange sequel, but the marketing for Doctor Strange 2 has shown he's more of a secondary antagonist it appears as a member of the Illuminati that seemingly has multiverse connections.
It might not seem like Doctor Strange's post-credits scene has anything to do with a multiverse Illuminati team, but it does quietly set up why Mordo is part of the roster in Doctor Strange in the Multiverse of Madness. After watching Strange break the laws of nature to stop Kaecilius and Dormammu, it makes sense that he could find himself as part of a council that oversees the entirety of the multiverse instead of focusing solely on sorcerers on Earth or in the main MCU timeline. Even though it isn't clear if the Mordo in Doctor Strange 2 is a variant from another universe or the same one featured in the first film, his progression off-screen to ing the team works either way. As a result, Marvel set up the Illuminati team six years before they debut in Doctor Strange in the Multiverse of Madness.
As a member of the multiverse Illuminati team in Doctor Strange 2, Mordo is in a position of power to expand on his mission from Doctor Strange's post-credits scene. The Illuminati are a group of Marvel's most powerful characters in the comics who work in secret to try and keep the universe safe. This included sending Hulk to a different planet out of an abundance of caution about his potential for destruction and pre-emptively attacking the Skrulls. These actions Secret Invasion storylines, which are rumored and confirmed as Phase 4 projects. Thanks to the great resources, knowledge, and power that the Illuminati contains, the MCU's version of this team could help Mordo achieve a greater goal than he previously imagined.
Mordo's mission in the Doctor Strange post-credits scene also explains why he could go after Doctor Strange in the sequel. The trailers have shown he gets arrested by the Illuminati and seemingly can't use his magic while in handcuffs. The Illuminati could step in after they see Strange mess with the multiverse, and Mordo recognizes the irresponsibility he continues to show with his magical powers. While the Illuminati might take Strange's punishment to a vote, Mordo likely won't be okay with seeing him potentially let off the hook in Doctor Strange in the Multiverse of Madness. Perhaps that is why they'll fight in the headquarters of the multiverse Illuminati team.
