I feel like there will be riots in the streets if RDJ isn't Tony Stark for the remainder of this Marvel Cinematic Universe. Don't replace him with another actor, just focus on a different hero. Quit trying to bleed these characters dry when there are plenty of other directions you can go in with Marvel characters. :smh:
The Iron Man films were so successful because Robert Downey Jr IS Tony Stark. Simple as that.
Assuming the Thor/Captain America sequels do well though, I don't see how Marvel couldn't just float the cash to keep these actors around for at least finishing up Phase 2. Once we get Pym and Strange involved I can deal with losing Thor and Iron Man, as long as they're just not around instead of being re-casted all together.