It's just perplexing. Even if we never see his face, we'll still hear his voice and think, "That's Tony Stark in there." on some level. Unless they modify his voice a bit, but then we come back to why do RDJ and not someone else? Which brings us back to stunt casting. And please, please, please don't come with a "twist" that he's an evil Tony Stark from another part of the multiverse and took on the name of Doom.