I don't know why Disney would spend tens of millions on Downey and
not make his Doom a Tony Stark variant, at least genetically. When Tony was an infant, Howard and Maria took him on a trip to Latveria or whatever, they were attacked, the kid was adopted, maybe even by people who had zero idea who he was, and he grew up to be Doom, with no real memory of his life before the incident.
That way, he's not meaningfully any kind of Tony Stark anymore, but someone completely different. And yet, once he realizes who his birth parents were, that provides all sorts of drama for him to undergo, as does the drama of characters who knew "our" Tony pleading with him to live up to the life of the guy whose life was taken from him.
Sure, they
could make Downey's Doom completely unrelated to Tony Stark, from an entirely different genome, but why throw away all that drama? That sense no make would. IMO.