My DOOMSDAY Theory (Probably not original)
My theory is that RDJ’s Doctor Doom is actually a Tony Stark variant who became Victor von Doom instead of Iron Man. The MCU has spent years establishing the multiverse and showing that variants can have completely different names, appearances, lives, and identities, so Victor genuinely being Victor von Doom doesn’t necessarily mean he can’t also be a Tony variant. Tony’s entire arc also makes this work thematically. In Age of Ultron, Wanda shows him his greatest fear, which is seeing the Avengers dead and being told by Steve that he could have saved them if he had done more. That fear is what pushes Tony toward creating Ultron and trying to build a suit of armor around the world. Tony has always had the ingredients that could create Doom: genius, ego, trauma, obsession with protecting Earth, and the belief that he sees threats other people don’t. Our Tony eventually learns that he can’t control everything and ultimately sacrifices himself to save everyone. An alternate Tony could reach the opposite conclusion and decide that the only way to save everyone is to control everything, which is basically Doom’s philosophy. The RDJ casting makes even more sense this way because Marvel could have cast anyone as Victor von Doom. Instead they brought back the actor whose face is synonymous with the MCU during the Multiverse Saga. It would also be weird as hell if Peter, Thor, Banner, Strange, and everyone else who knew Tony saw Doom’s face and nobody acknowledged that he looks exactly like Tony. Imagine Peter seeing Doom unmasked and thinking Tony somehow came back, while Doom has no idea who Peter is. Then Doom eventually learns that another version of himself existed as Tony Stark, was loved by the Avengers, saved the universe, and sacrificed himself doing it. That would make Tony and Doom two completely different outcomes of the same person. Tony believed saving the world eventually meant sacrificing himself. Doom believes saving the world means everyone else sacrificing their freedom. That gives RDJ playing Doom an actual narrative purpose instead of just being stunt casting. It isn’t confirmed that Doom is a Tony variant, but with the multiverse, Tony’s established character arc, and Marvel deliberately bringing RDJ back as the main villain, I think there is a strong chance the connection is intentional.