Robert Downey Jr. is making a surprise return to Marvel as Doctor Doom in 'Avengers Doomsday.' Here's how that's possible and what else to know about the film.

The end of "Loki" season two seemingly diminished Kang's role in the MCU when Tom Hiddleston's titular antihero rewrote the timeline. This provided an easy route to eliminate Kang as the main villain.

Now, Marvel has pivoted to casting Downey Jr. as Doctor Doom. Though Downey Jr. is not the first actor to play multiple characters within the MCU, he'll become the first to play two major characters.

The answer to how this is possible lies in the multiverse — various dimensions/timelines that are connected together.

Recent Marvel projects have introduced fans to alternate versions of heroes from other universes including Doctor Strange (Benedict Cumberbatch), Scarlet Witch (Elizabeth Olsen), Deadpool (Ryan Reynolds), Wolverine (Hugh Jackman), and more.

So Downey Jr.'s Doom could easily come from another universe to distinguish him from Iron Man. And the villain wears a metal mask, after all.

Doom is the Fantastic Four's main enemy in the comics, and Marvel Studios head honcho, Kevin Feige, already confirmed that the team's movie, "The Fantastic Four: First Steps" takes place in another universe. Perhaps Tony Stark never got the chance to step up as a hero and chose a darker path in that world.

There have also been versions of Iron Man in the comics that have turned into Doom. Most notably, a young Victor Von Doom body-swapped with Stark as a young man in 2010's "What If? Iron Man: Demon in an Armor #1."

Doom himself even took up the Iron Man mantle in "The Infamous Iron Man" series in 2016. So, there is some precedent for blending their identities.

If the villain does come from another universe, Marvel Studios could still cast another version of Doom for the main MCU on Earth-616 if they wanted someone else to take over the role in the future.

The internet is divided about Downey Jr.'s return, with many criticizing Marvel Studios for playing the nostalgia card and being too safe with their casting choice.

But Rob Mitchell, the director of theatrical insights at Gower Street Analytics, told Business Insider that bringing him and the Russo brothers back is a logical decision.

"The chances are people are going to go, 'This is interesting. What are they doing? I've got to see this. Even if I just want to be able to have an opinion on why.' I get it from a business perspective, I can't really see the downside," he said.

Mitchell added: "Fans would've expected big things from an 'Avengers' movie anyway. And of course, this is the 'Avengers' movie prior to 'Secret Wars.'"

Mitchell noted that the curiosity about Downey's role in "Doomsday" and "Secret Wars" will draw people in, especially to see if the actor changes his appearance.

"Will he look like Tony Stark or will they do something to make him look different? It's Hollywood, there's plenty of makeup. There's lots of Ryan Reynolds in 'Deadpool & Wolverine' but they don't all look the same."

Downey Jr.'s casting comes amid conversations about whether audiences are tired of the superhero genre. While it's not a new concern, Mitchell noted that MCU fatigue hasn't deterred fans.

"Can we really have a conversation about MCU fatigue when 'Deadpool' just made $430 million in four days? This is a film that we are anticipating being only the second R rated film to do a billion dollars. And clearly, that's where it's heading. So you can talk MCU fatigue after that," he said.

Michael Waldron, who wrote "Loki," will be writing the script for the two Avengers movies, which means the film will likely be related to that series.

"Loki," "What If," "Doctor Strange in the Multiverse of Madness," and "Ant-Man and the Wasp: Quantumania" warned that multiversal interactions could lead to a war between universes or the universes colliding into each other.

Though Kang is no longer Marvel's big-picture villain, a collision between different worlds still seems to be the Avengers-level threat that will be faced in the next two films.

Marvel seems to be adapting two major events from Marvel comics in 2015 and 2016, "Time Runs out" and "Secret Wars. In this run of comics, both written by Jonathan Hickman, the multiverse is dying because an alien race called the Beyonders has created living bombs to wipe out everything.

Attempts to kill living bombs lead to multiple earths from these universes colliding, wiping out both universes. The Beyonders' plan succeeds, but Doctor Doom, Doctor Strange, and Molecule Man kill the alien race, steal their power, and create a new world with Doom as the world's god.

Other heroes also survive the decimation using special life raft machines. When they escape, they defeat Doom and restore the universe.

Incursions have already been alluded to in "Doctor Strange in the Multiverse of Madness." While they have the same effect as the comic book, the cause is prolonged multiversal traveling.

It is likely that something before or during "Avengers: Doomsday" kicks off that incursion in the main MCU universe, leading to a battle between multiple worlds. So fans may see even more cameos in the movie outside Downey Jr.'s Doom from one of the other worlds.

Since "Avengers Endgame," many of the main heroes from previous films have retired or died, meaning there may be new heroes in the upcoming film.

Sam Wilson/Falcon (Anthony Mackie) has taken over as the new Captain America and has his own sidekick, Falcon (Danny Ramirez). They will likely be leading the new Avengers alongside other known characters including Doctor Stranger (Benedict Cumberbatch), Wong (Benedict Wong), Captain Marvel (Brie Larson), Thor (Chris Hemsworth), Spider-Man (Tom Holland), and the Hulk (Mark Ruffalo).

Ant-Man (Paul Rudd), the Wasp (Evangeline Lilly), and Cassie Lang (Kathryn Newton) have already faced a multiversal threat, Kang, in "Ant-Man and the Wasp: Quantumania" so they will likely also help out the Avengers team.

The original Guardians of the Galaxy disbanded after "Guardians of the Galaxy Vol.3." Still, the new team featuring Groot (Vin Diesel), Rocket (Bradley Cooper), Kraglin (Sean Gunn), Cosmo (Maria Bakalova), Blurp, Phyla (Kai Zen) and Adam Warlock (Will Poulter) could appear.

It is unclear what Marvel plans to do with Black Panther and Wakanda, but they will probably help out in some way, along with the Namor and the Atlanteans (Tenoch Huerta).

In recent films, Marvel has introduced multiple new characters, such as The Eternals, She-Hulk, Ms. Marvel, Daredevil, Deadpool, and Shang-Chi. They will likely be tied into the film at some point.

The only actors that Marvel has confirmed will appear in "Avengers: Doomsday" are the cast of the upcoming "The Fantastic Four: First Steps" and "Thunderbolts" movies.

"Fantastic Four," starring Pedro Pascal, Vanessa Kirby, Joseph Quinn, and Ebon Moss-Bachrach, follows a group of astronauts who get amazing powers after a mission goes wrong.

Meanwhile, "Thunderbolts" stars a group of assassins and spies fans have met in previous films and TV shows that must work together on a mission.

The cast includes Winter Soldier (Sebastian Stan), Yelena (Florence Pugh), John Walker (Wyatt Russell), Red Guardian (David Harbour), Ghost (Hannah John-Kamen), Taskmaster (Olga Kurylenko) and Valentina Allegra de Fontaine (Julia Louis-Dreyfus). Lewis Pullman also stars in the film as a new hero, Sentry.