Of the Avengers we have seen thus far RDJ as Tony Stark has had the most face time. Plus, you would be blowing up the great chem Stark had with Bruce Banner 3.0. (or would it be 4.0 or 4.5 considering other actors?)![]()
As far as the MCU is concerned Mark Ruffalo is only Banner 2.0
3.0 - before him there was Eric Bana and Ed. Norton.
But as far as the Marvel Cinematic Universe is concerned Ruffalo is only the second Banner/Hulk. The Bana movie is not part of the Marvel Cinematic Universe.