Theory: Dr. Doom (RDJ) is a gestalt entity (detailed theory in body text)

[u/ghostrider8303]

The Dr. Doom we would meet in Avengers: Doomsday comes from a universe where he developed his time platform and was ready to explore and manipulate the timeline to his benefit when Latveria was invaded by that universes Avengers led by Iron Man to arrest him. The battle would lead Iron Man and Dr. Doom to both fall through the time platform and merge into a single entity with Tony Stark's face but Dr. Doom in control. The gestalt Dr. Doom can now freely travel in time across various universes and his activities leads to several incursions occuring.

The final battle of Doomsday could very well come down to the Avengers figuring out a way for Tony Stark to wrest control over their common body from Doom.

[u/Impressive-Handle-69]

I like the concept, but Tony Stark being adopted(vaguely hinted at in Endgame 1970 elevator scene Tony Stark seems puzzled by the news of his mom expecting soon) would make more sense. We'd be getting a Dr. Doom that was never adopted by the Starks. This could be inspired/altered from the comics where Tony discovers he was adopted and the Starks have a biological son named Arno Stark.

My rabbit hole theory that ties Kang into this would be that the 'Sacred Timeline' is our timeline that Kang created and set up to:

1: Create a timeline where he(Kang) is not born, to help prevent the council of Kangs(no loose ends)

2: Create a timeline where Victor Von Doom isn't evil/becomes Dr Doom.

Both of which also means no Fantastic 4, which would explain why we don't see them in OUR univers(their movie takes place in a separate universe). Victor doesn't meet Reed, leading to the events of them all getting their powers.

With Kang out of the picture, and Loki sitting on the throne, this means that universe can now exist.

Bonus theory: Everything same as before, plus the added; This is the Dr. Doom from the universe featuring the Illuminati in Dr Strange the Multiverse Of Madness, where Wanda single handedly eliminated the Illuminati. Dr. Doom was off studying the multiverse, explaining an empty chair. Dr Doom returns, discovers everyone is dead, and begin to seek out revenge, discovers if other Universes/timelines can harm his, he must eradicate all others and makes 1 single world where he rules and protects with absolute authority. If it happens to be God Emperor Doom with the power of the Beyonder, that'd be totally bad a$$.