Downey is an accomplished, gifted, award-winning character actor, which is more than enough reason to cast him in anything. He's also someone the producers know well and apparently liked to work with. Plenty of film and TV creators have actors they like to work with over and over again, like Sam Raimi using Bruce Campbell in everything or Tim Burton using Johnny Depp in countless things. But Marvel Studios is in the unusual situation of having all their films be in the same universe, so if they want to bring back an actor they like after their character died, it would have to be as a different character in the same universe. Doom is a good choice for that, because his face is rarely seen, and if it were, it would probably be heavily scarred, so the actor's appearance would be a non-issue.
Of course, we can't rule out the possibility that they do intend some connection to Stark. But one should acknowledge
every possibility that hasn't been ruled out, and we can't rule out the possibility that it's not about the character but about the actor, his skill and his relationship with the producers. At least, I'll be much happier if Doom isn't connected to Stark.
The term "metahuman" is not exclusive to DC. It was popularized by the
Wild Cards shared-universe novels and RPG developed by George R.R. Martin in the 1980s, but there are
citations for it in fiction in the 1970s and in critical works in the 1960s. It just means someone beyond human, posthuman, or transformed from human, depending on which definition of "meta-" you choose. It actually has been used in a Marvel context from time to time:
https://archive.org/details/spidermanamazing0000tobi/page/n49/mode/1up