I think that Iron Man will be completely done with the superhero game after Age of Ultron. He already kind of gave up the Iron Man gig after Iron Man 3, when he had the arc reactor removed from his chest. However, he is still creating new suits, which implies that he is not completely done. Because Ultron is mostly his fault, I believe that Iron Man will give up the science and superhero gig forever, (until Civil War). My prediction is that he will immediately either return to Malibu, with no intention of rejoining that world of superheroes. Or, he will join SHIELD/the Government, thus setting up his ideals for Civil War.
Captain America is supposedly going to be assembling a new team of Avengers after AoU. If this true, the I believe that he will assemble: Falcon, possibly the super twins, Black Widow, possibly a young Spider-Man, and maybe even a very reluctant Black Panther. If not, I believe that he might rejoin SHIELD, but that seems very unlikely, seeing as he and SHIELD's ideals have not been very compatible in the past. However, if the "real SHIELD" from [Agents of S.H.I.E.L.D.](series:722469), the tv show, takes over, maybe Captain America will join them. They both share the ideal that secrets are terrible when it comes to protecting the world, so maybe they will hit it off nicely.
Bruce Banner/the Hulk, is rumored to end up off world after AoU, potentially setting up the Planet Hulk storyline. I think that would be great, but extremely risky. That also fits into the rumors that Hulk could be joining the Guardians of the Galaxy in their sequel. This would explain why Marvel executives said that James Gunn's first pitch for GotG 2 was risky. If not off world, I think that the bulk could possibly back into hiding, maybe going to the American western deserts like he so often does in the comics. A third option is that he surrenders himself to SHIELD/the Government, so that he can never again return to his rampaging ways.
Black Widow has not had her solo film, and there is not one in the phase 3 plan (as far as we know). So, if she doesn't get her won solo film in the MCU's phase 3, then the most plausible thing to me is for Captain America to recruit her for his New Avengers. If Black Widow doesn't join Cap's new team, then the only other thing that I can think of is that she will continue to be Nick Fury's top spy. In the comics, Widow is known for being Fury's lap dog, and I think that could play into Marvel's phase 3 nicely. Maybe her role in [Captain America: Civil War](movie:994409) will be to try and keep everyone as cooled off as possible, under direct order from Fury.
My theory for Hawkeye is that he will most likely join Iron Man away from the superhero/spy game, and possibly going to work for Stark. With the rumors that Hawkeye and Maria Hill will possibly have a little romance brewing between them, it seems plausible to me because Hill is already working for Stark. The other theory is that Hawkeye will join either the old SHIELD, with Fury at the head, or the "real SHIELD," with the no secret policy.
The super twins are new to the Avengers, but I think that they will fit in nicely. First, Quicksilver. My theory for him is either that he will be the one to die at the end of the movie, or he will be joining Cap's New Avengers. My reasoning for his death is because a recurring moment in AoU trailers is a scene where Scarlet Witch completely breaks down and screams he head off. The death of your twin would definitely be cause for a little emotional breakdown. Of course, this could just some of the emotional problems that Scarlet Witch shows repeatedly in the comics. My theory for Scarlet Witch is that she survives, for two reasons. First, she has the power to potentially do anything that she wants, much like Jean Grey of the X-Men. Second, she is already rumored to be In Captain America: Civil War. If she does survive, then I'm guessing that she will also join Cap's new team.
Basically, the only theory that I have for Nick Fury is that he will survive, because he always does, and will continue to do his Nick Fury thing.
Maria Hill is currently working for Stark Industries (possibly under orders from Nick Fury), and my prediction is that will not change. But if is does, she will most likely rejoin Fury.
The Vision is, for me, personally, the most anticipated new character in AoU. I love the way he looks, and I can;t wait to see if he really is powered by the yellow infinity stone. The Vision is rumored to be the one to take down Ultron, which makes sense if he is powered by an infinity stone. My theory is that he will either sacrifice his life to take Ultron down, or he will live on. There are also rumors that he will join Iron Man in Civil War, but those are far but confirmed.
I bet you are wondering why Thor has been absent until this point. Well, that is because I think that Thor will be then to DIE in AoU. Here's why. Scarlet Witch will reportedly give everyone visions of their greatest fears, and for Thor, that is Ragnarok (apocalypse). His death makes sense because in the Civil War storyline, Thor is "dead." Also, that would set up Ragnarok nicely, because, as far as I know, Thor is also dead to start off that storyline. Another reason for his death is because he, being Asgardian, can be brought back to life pretty easily, while the other Avengers can't. If Thor doesn't die, then I think that he will return to Asgard to try and be there to either stop Ragnarok, or to end it as soon as possible.
There are my theories about what could happen to all off the Avengers by the end [The Avengers: Age Of Ultron](movie:293035).