According to Badass Digest, it’s possible that Marvel could sit out characters like Captain America, Thor, and Black Widow for Avengers 3. Robert Downey Jr‘s contract specifically states he has to be in Avengers 3, so Marvel could use Iron Man as the anchor for new characters or second-stringers who need a bit of a boost. Meanwhile, Cap and Thor wouldn’t have to be thrust back into an Avengers movie following the fallout of their respective third solo adventures, and Black Widow could be off on a mission somewhere. Badass even hints that Hulk may not be on Earth by the time Avengers 3 rolls around.
Marvel’s power is now in the brand, so audiences might be willing to go along with the fact that the team they rooted for in the first two movies is different, especially if Iron Man is still there to ease the transition. Additionally, it’s good to rest some of the characters and especially the actors as Badass notes that it might help soften them up for renegotiations.
Where it gets really interesting is what Avengers 3 could lead to the following year. If Cap, Thor, et al. sit out of the third Avengers movie, then they could come back for something truly massive. Badass speculates that Avengers 3 isn’t the conclusion of the Thanos storyline, but the “spark” that ignites “a huge, all-franchise crossover” bringing together all of Marvel’s characters including the ones on television. That would be some damned impressive planning.