I'm not going to get too wrapped up in the Stark/Doom stuff.
I see this as two basic scenarios.
1. A play on a variant version of Tony Stark that becomes "Dr. Doom", and is a total mind screw to Peter Parker and others. There's a lot of character acting that can go on there.
2. He's Victor Von Doom, and is a 'variant' so he just happens to look like Stark. or, they wont' really use his visage at all.
RDJ is a very good actor and can pull this off. We shall see. 2 years off, right?