Originally Posted by Visiting Arkham
Honestly, get him on for Avengers 2 and 3 (probably the last ones Joss Whedon will be on board for) and then let him go. No more solo movies, Phase 3 can consist of plenty of films that aren't Iron Man, and we can have RDJ play Tony Stark through 2018ish. Think about how special it will feel having a 3 year gap between the second and third Avengers waiting for the final appearance of Downey Jr. as Stark, that will be an event in itself. That way, he can exit the franchise on a high note (while surely getting a boatload of cash for appearing) and they can make it an epic swan song for the character. It's a lot like serialized television, like LOST or Breaking Bad, having an end date improves the storytelling tremendously.
Who knows what Marvel wants to do after The Avengers 3 and into Phase 4, but by then RDJ will have appeared as Iron Man 6 times in 10 years, and hopefully 5 out of the 6 will be great, with one appearance being merely pretty entertaining. That's all you can ask of the man at that point, especially considering he is arguably the second or third guy responsible for the success of Marvel Studios behind Kevin Feige and maybe Jon Favreau. Let him exit the universe gracefully, and allow new talent to filter in to build a new path in the Marvel Studios universe.
I really do think this is the way Marvel/Disney intends to go. While the Iron Man character both literally and figuratively spearheads the Marvel Cinematic Universe, I believe IM3 will aim to wrap up 'Tony Stark' specific Iron Man themes and story arcs. I believe we'll see Stark again in Avengers 2 (obviously), followed by Avengers 3, whenever that's planned.
If we see Stark at all in the Phase 3 films, I believe it will be via references and/or cameo appearances from Downey... which, I believe is the way to go.