Spoiler alert: This article contains spoilers for Doctor Strange in the Multiverse of Madness and various other MCU products.
The Marvel Cinematic Universe has already begun setting up its next Endgame-level threat for the Avengers, though the nature of this dilemma has remained a highly guarded secret for some time. Since the close of the Infinity Saga, Marvel fans have been left to wonder what this next big crossover might be.
While there has not yet been any word from Kevin Feige or the executives at Marvel Studios, many fans strongly believe that the MCU might be heading toward a Secret Wars event, a paradigm-shattering multiversal collision from the comics wherein the greatest heroes from multiple earths are placed on an ethereal mass known as Battleworld, leading to a major incursion.
The Multiversal Catalog
Doctor Strange in the Multiverse of Madness has only furthered fan theories that a Secret Wars crossover could be on the horizon, with the film's ending implying that the MCU was just one of many universes about to collide with another. This is especially evident in Earth-838 Christine Palmer's catalog of different Earths that she has encountered.
Christine's index of the earths that she has encountered implies not only the existence of dozens of other realities but also that inter-universal travel is far more common than fans initially thought. While this information has endless implications for the franchise's future, it also implies that incursions will grow in commonality as the various realities interact with one another.
The Russo Brothers' Comments
The Russo Brothers are two of the most prolific directors to ever work with Marvel, helming four of the franchise's biggest hits, including Avengers: Endgame, which is the MCU's highest-grossing film to date. While the directorial team has publicly stated that they believe their time with Marvel to be finished, they have also admitted that a Secret Wars event would be what could bring them back to the fold.
While the MCU has no issues in the financial department, it would be foolish of Marvel to ignore the directing duo that has become one of their most reliable moneymakers. Joe and Anthony Russo were at the helm of three of the ten highest-grossing MCU films of all time so far, and have already proven that they can balance sprawling casts, which a Secret Wars crossover would inherently include. As such, the Russos' statement bodes well for those hoping to see the infamous comic crossover.
Miles Morales References
One of the most famous consequences of the 2015 Secret Wars storyline was the arrival of Miles Morales, formally the Spider-Man of Marvel's Ultimate Universe, into the mainline Earth-616. With this knowledge, fans have especially perked up at several sly references to the character in Spider-Man: Homecoming and No Way Home.
Many have wondered when the MCU would bring the fan-favorite character of Miles Morales into the fold, and, with the increasingly common references to his character within the franchise, it seems as though Marvel Studios is attempting to hint at his future therein. If this is so, a live-action version of Miles could be in the near future for the MCU.
Incursions
Multiverse of Madness introduced this concept of incursions, explaining that one occurs when two parallel universes collide, destroying one or both of these realities in the process. Moreover, the film's mid-credits scene ended with the implication that Earth-616 is headed for an incursion that may not be avoidable.
An incursion seems inevitable for Earth-616, which could take the form of a Secret Wars event wherein the strongest heroes from two worlds are forced to battle for the survival of their realities. Though the exact universes with which the MCU has an incursion remains up for theorizing, fans can expect the possible crossover to be the result of this collision between differing realities.
The Mutant Problem
Many fans expected Marvel's mutants to be introduced in Multiverse of Madness, but this theory ultimately proved to be incorrect. As such, there remains a lingering question for Marvel fans: how will mutants be incorporated into the MCU? After years of theories, Secret Wars might be the perfect solution to the growing mutant problem.
With the introduction of the concept of incursions to the MCU, Marvel seems to have already revealed the method by which it will incorporate the mutants. Having Earth-616 collide with an Earth already rife with mutantkind, perhaps Fox's own previously established franchise, would allow for the mutants to be seamlessly inducted into the MCU, leaving the world with both the Avengers and the X-Men teams moving forward.
The Sacred Timeline
The Disney+ series Loki did plenty to encourage fan theories that Secret Wars is on the horizon for the Marvel Cinematic Universe. One way in which this was done came as early as the show's first episode: with the introduction of the Sacred Timeline.
The Sacred Timeline is a sequence of events that the TVA believes to be dictated by the Timekeepers. However, this is revealed to be a ruse created by He Who Remains, a variant of Kang the Conqueror. This lie may prove significant, however, it still implies that there is something special about the MCU's timeline that differentiates it from others, perhaps indicating that it must extinguish the very existence of other timelines in order to save the multiverse as a whole.
Multiversal War
Loki introduced the concept of multiversal travel for the first time in the MCU but also warned of how perilous such an action can be: revealing that, at some point, there was a war between different universes that nearly ended reality in totality.
The mention of a multiversal war should have been enough to perk the ears of any Marvel Comics fan, as this is a very general description of Secret Wars itself. Many have even theorized that, given the ever-looping nature of time and reality, this "multiversal war" could refer to an event yet in the future, which will result in the creation of the TVA, He Who Remains, and all the other events seen in Loki.
The Rings
Shang-Chi is one of the best Avengers introduced in Marvel's Phase 4, in part due to the ancient and mysterious Ten Rings that he inherited from his father. The mid-credit scene of the character's debut film revealed that the Rings were created from an ancient and unknown material, which was now calling out to something as-yet-undiscovered.
This scene may indicate the coming of the Beyonder, the supreme entity which initiated the events of the original Secret Wars storyline in 1984. In this storyline, it was the formation of the Molecule Man that alerted the Beyonder to Earth-616's existence. Perhaps the MCU's reawakening of the Ten Rings will serve as the inciting incident to grab the attention of this cosmic force.
The Death Of He Who Remains
In the finale of Loki, Loki and Sylvie meet He Who Remains, who claims that the multiverse will begin to cave in on itself should they kill him. Unlike Loki, Sylvie takes the bait, killing the variant of Kang the Conqueror, thus destroying the flow of the multiverse that he had set up, creating unknown levels of chaos throughout reality itself.
While the true effects of Sylvie's actions remain unexplored, Kevin Feige has confirmed that the events of Multiverse of Madness are directly tied to the end of Loki, implying that the death of He Who Remains could spell certain doom for the multiverse, one that could only end in a destructive incursion that Secret Wars would undoubtedly bring about.
The Coming Of Kang
While very little has been officially revealed about the next Avengers-level threat in the MCU, it seems incredibly likely that it will have to do with Jonathan Majors' recently revealed Kang the Conqueror, who first appeared in the finale of Loki, and will make his next appearance in Ant-Man and the Wasp: Quantumania next year.
Kang is a character intimately connected with the notion of time travel and the multiverse in the Marvel Universe. The character's importance in the next phases of the MCU seems to denote that he could take the place of Doctor Doom from the comics, creating his own version of Battleworld, leading to a Secret Wars adaptation for live-action that fans won't soon forget.