I'm kind of weirdly excited because I've been wanting him to come back as Doctor Doom for awhile now but didn't think it would actually happen.
There's just so much room for character drama with our avengers having to make sense of a villainous version of their hero. Also room to play with the dichotomy of stark being a man of science and Doom more into magic.
Is there a reason you wanted RDJ for Doctor Doom? I haven't read the comics so I'm unfamiliar but I would have expected him not even to be in the conversation. Is there some kind of cannon connection between stark and him? I always thought his link was to the Fantastic Four primarily.
There was a short comic run where Dr. Doom becomes Ironman after Tony dies that I really liked. The only thing connecting them is that they're both dudes in suits of armor otherwise.
There's a "What If...?" comic where Tony, not Reed Richards, is Vic's college roommate and Vic ends up stealing Tonys identity, literally a "Freaky Friday" Marvel version.