What character should Robert Downey Jr. play on Doctor Who?

A rich and wealthy businessman, a human. A funny, witty, boy of a man. But he sees the world and wants to make it good. So he builds a robotic suit, one that can fly and shoot lasers. And maybe he meets others, like a WW2 Captain thrown out of time, an alien who has the appearance of a god, and a mutant. Oh wait... But seriously, I would like to see him play some sort of baddy that has power. Something similar to Van Staten but more sinister, someone actively being 'bad'. Not someone who does his own dirty work but controls things from afar. Or rather, I would not like him in it at all. He's too big, too famous for me. I like my Doctor Who guest actors to be less so, less risk of breaking the immersion. Even Frank Skinner was a bit too famous to be in it. But maybe that's just me.