If Marvel actually explains RDJ’s face in-universe, I see 3 possibilities:

1. Victor weaponizes the resemblance.

He’s 100% Victor von Doom, but discovers Earth-616 Tony looks exactly like him and uses that against the Avengers. He doesn’t need them to believe he’s Tony. He just needs them to hesitate.

2. Victor is Earth-828’s Arno Stark.

He’s that universe’s Tony Stark’s biological brother, separated from the Starks and raised by the von Dooms. He’s still genuinely Victor, but this explains the resemblance. And once he learns what Tony meant to the Avengers, #1 could happen too.

3. Tony died, but his consciousness ended up inside Victor.

Tony really dies in Endgame, but using all six Stones sends his mind into Earth-828’s Victor.

Sue says Victor used to be a good man. Maybe she’s remembering Victor before Tony entered him. Victor’s feelings for Sue could then collide with Tony’s memories of Pepper.

Tony’s Snap injuries could also explain Doom’s disfigurement, with Franklin potentially being the person capable of restoring the face Tony remembers having.

And Wanda’s warning in Age of Ultron finally pays off: Tony could lose sight of the difference between saving the world and destroying it.

Even Doom’s “stolen lives” line fits: Tony gave everyone else their lives back... then woke up living Victor’s.

A, B, C, or none?

3 reasons Doom could look like Tony Stark