At this point you have to negotiate with him on a movie to movie basis. Anything long term, 2+ movies, and Disney would have to break the bank. Plus RDJ may not want another big commitment. I would get him back for Avengers 2 but provide some sort of conclusion to the character. Let him leave the armor behind for the next generation. Recasting the character or "rebooting" would be kinda lame, especially if they nabbed an actor ten years younger.