I think Tony Stark was Victor Von Doom all along.

I think the Tony stark we know of in the MCU was actually Victor Von doom all along, but was raised by the Starks. The real Tony Stark was a miscarriage, and Victor, (who is an orphan in the comics) was adopted by the Starks, grew up to be Iron man instead of Doctor Doom and of course gave his life to beat Thanos and save half the universe.

Doom and Tony stark share quite a few characteristics, (driven by Ego, Genius intellect, master inventors, strong urge to lead others - either the avengers or Latveria) so it’s plausible that Victor could grow up as Tony.

I think the introduction of Doom will be a variant that wasn’t adopted by the Starks, who grew up to be the Doom we know of, in Latveria. This could explain why they will be identical, and could open up the opportunity to bring in a new iron man who was the real Tony Stark, played by someone else.

I think it would be much cooler twist than the idea of Doom being a Tony Stark who just turned evil for some reason. Or just that Doom will look exactly like Tony for no reason other than he’s also played by RDJ. Both of those just seem way too simple and obvious, I think Marvel know that we are expecting this so they’ll probably try and do something that most people would never expect.

Probably far fetched, and I’m not sure how people would feel about RDJ technically not being the true Tony stark with this theory. But I honestly think it’s quite cool.