In the Marvel Comics universe, there’s a storyline where Tony Stark’s consciousness gets transferred into Victor von Doom’s body, effectively making Stark the new Doctor Doom. This plot is explored in the "What If...?" series, specifically in issue #33, where Stark, now in Doom’s body, becomes a Knight of the Round Table in an alternate timeline.
However, in the mainstream Marvel Cinematic Universe (MCU), Robert Downey Jr. is set to portray Doctor Doom, which is quite a shift from his iconic role as Tony Stark/Iron Man.
At the moment, there’s no official word on Robert Downey Jr.’s contract or how much he’ll be paid for playing Doctor Doom in the upcoming MCU films. While some reports suggest that his earnings could break records, official confirmation has yet to be made.
