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That would be a bit cheap, the best bet is this Doom is an alternate reality Iron Man.

Entertainment Weekly have been trying to piece the clues together:

Marvel directors Joe and Anthony Russo will be re-teaming with Downey for the first time since they killed off Tony Stark together in 2019’s Avengers: Endgame, but they did not divulge many details about their upcoming movies other than their titles. The fifth Avengers movie, initially called Avengers: The Kang Dynasty, has now been revamped to Avengers: Doomsday with a focus on Doom instead of Kang, while the sixth one will still be Avengers: Secret Wars. But the Russos did mention that introducing Doom into the MCU is essential preparation for the latter film, which gives a slight indication of what we’re in for here.

"It's the biggest story that Marvel Comics ever told," Joe Russo said on stage about Secret Wars. "It's the reason Anthony and I are standing up here. It's world-colliding epic-ness. But we need to make another movie first. There is one very important character that is required to do it justice."

Secret Wars was a massive Marvel event comic, written by Jonathan Hickman with art by Esad Ribic, and first published in 2015. It tells an epic story of the multiverse getting destroyed by a process of parallel universes colliding into each other, thanks to the machinations of a group of powerful supernatural beings called the Ivory Kings. In that comic, Doom is able to save a small group of characters from across time and space and assemble them on a ragtag planet called Battleworld.

In other words, you can’t do Secret Wars without Doom, and if the post-credits scene of The Marvels is any indication, we won't see an MCU X-Men movie without Secret Wars bringing parallel universes together.

Since Secret Wars is a multiverse story, that raises the distinct possibility that Downey's Doom may by a multiversal variant of Tony Stark. That would be different than how variants have previously worked in the MCU, where they all share a common name or (in the case of the Deadpools from Deadpool & Wolverine) at least a costume. But then, Doom never plays by anyone else's rules.

Introducing Doom in an Avengers movie is another surprise, since the character is usually the archenemy of the Fantastic Four — who, as it happens, are also making their long-awaited MCU debut soon. That movie is set to feature Ralph Ineson's world eater Galactus as the primary adversary of the team, but since it's set in a different universe than the main Avengers timeline, maybe Doom is that world's equivalent of Stark. The fact that we'll now have seen the character Reed Richards played by two different actors in MCU movies (John Krasinski in Doctor Strange in the Multiverse of Madness and now Pedro Pascal in The Fantastic Four: First Steps) may make it easier to accept the same actor playing two different characters.

So Secret Wars will see a lot of multiverse variants brought together by Doom, so it opens the door for him to actually be an alternate Iron Man. It may also be the key to bringing the X-Men and FF into the MCU or making it more viable to cross them over somehow.

It may even be we'll see RDJ in the FF film, perhaps just a cameo or end-credit scene, as he would need some set-up before Avengers: Doomsday.