This is the most widely discussed theory — and for good reason. The casting of Robert Downey Jr. as Doom after a decade as Iron Man is too pointed to be coincidence. The MCU's own rules, established in Loki and What If...?, confirm that variants of the same person can look identical across universes.

The theory goes like this: Doom is a version of Tony Stark who never had his humbling crucible moments — no cave in Afghanistan, no Pepper, no Morgan, no arc reactor in his chest. Without those experiences that ground the MCU Tony, this variant became everything Tony's worst instincts wanted: arrogant, controlling, and convinced that he — and only he — has the intelligence to fix what others broke.

This also explains the emotional engine of the film. Imagine surviving Avengers — Rhodey, Steve Rogers, Pepper — looking into Doom's face and seeing Tony. That's not a gimmick. That's a narrative weapon.

Incursions — universes physically colliding and annihilating each other — were introduced in Doctor Strange in the Multiverse of Madness. Deadpool & Wolverine expanded on the concept of "anchor beings": individuals so tied to their universe's identity that their death destabilizes the timeline around them.

The theory: Tony Stark's death in Endgame was Earth-616's anchor being event. It saved the universe in the short term but fractured the multiverse's stability in the long term — because the most important person in that reality sacrificed himself out of sequence. The cracks have been spreading ever since Endgame, through No Way Home, Multiverse of Madness, and now Doomsday.

Steve Rogers' decision to stay in the past with Peggy is also theorized as a contributing factor — branching the timeline deliberately in a way that has cascading consequences.

Doom positions himself as the only one willing to make the impossible choices to stop the collapse — much like Thanos did, but with greater intelligence and arguably more coherent logic.

One of the most structurally bold predictions: Doomsday is Doom's movie, told primarily from his point of view — exactly the way Infinity War was Thanos' film. The heroes are obstacles, not protagonists.

In this framing, Doom is not a cackling villain. He is a man of absolute conviction who believes — and is often correct — that the world's chaos requires a single intelligence in control. His arc across the film involves acquiring power systematically, making brutal but logical decisions, and ultimately positioning himself as the necessary evil that the heroes cannot bring themselves to be.

Marketing imagery has suggested Doom physically defeating Thanos — possibly tearing the Mad Titan's spine out — as an early power statement. This would establish immediately that Doom is not simply "the next Thanos." He is on a different level, and the film's dramatic question is not whether the heroes can beat him physically, but whether they can out-think him morally.

Introduced in The Fantastic Four: First Steps, young Franklin Richards possesses reality-warping powers that in the comics are considered among the most dangerous in existence — he can create fully functional pocket universes. In Secret Wars comics, it is Franklin who provides Doom with the raw creative power to become God Emperor of the patchwork world called Battleworld.

The theory: Doom's central goal in Doomsday is to acquire Franklin — not out of cruelty, but because Franklin's powers are the only force capable of either stabilizing the collapsing multiverse or creating a new singular reality from its remnants. Doom frames this as stewardship. The Fantastic Four — and eventually all the heroes — see it as a child being weaponized by a tyrant.

This sets up Secret Wars perfectly: Doom wins partial control by the end of Doomsday, and the sequel deals with the consequences of a universe remade according to his vision.

With legacy X-Men actors confirmed returning — Patrick Stewart as Professor X, Ian McKellen as Magneto, James Marsden as Cyclops — this is not really a theory anymore. The X-Men are in this film, coming from a different universe. The heroes from different realities meeting will produce conflict before cooperation. This is both dramatically necessary and structurally expected from the Russo brothers' playbook.

The more specific theory: the MCU Avengers and the Fox-universe X-Men have fundamentally incompatible world-views at first contact. Mutants have lived in a world that feared and hunted them. Avengers have operated as celebrated public heroes. That friction does not dissolve immediately just because Doom shows up. It creates a multi-faction crisis that Doom may actively be exploiting — keeping the heroes divided while he moves toward his goal.

This is the most morally provocative theory, and the loosest in terms of hard evidence. The idea: Doctor Strange, Reed Richards, or a secret group of elites (a "Council of Reeds" or Illuminati-type organization) conclude that Doom's plan — though authoritarian — is the only functional solution to total multiversal collapse. So they do not stop him. They let him win. They may even help him.

The emotional weight would be immense: heroes choosing surrender to a tyrant because the alternative is the end of everything. It sets up Secret Wars as a resistance story against a reality that was deliberately handed over.

The reason this ranks last: it requires complex setup that has not been confirmed, and it risks being too similar to the "let Thanos win" structure of Infinity War. The Russos are smart enough to avoid straight repetition. Still — as a thematic thread within a larger plot, it has real power.