Scarlett Johansson has already said that she wont be in Iron Man 3, but she would love to do a solo Black Widow film and make it like a Bourne Identity type film. So I think she would also refuse to play second banana to Captain America in his solo film too. And Renner is taking up the Bourne franchise too, so I doubt he will want to continue playing second banana as well.
I think they are probably signed up for some solo films for their character in case the public wanted to see a solo Hawkeye movie and a solo Black Widow, and I am sure they are signed for at least 3 Avenger films. But I can only see them showing up in other super films if they have it in their contract that they had to. I have heard Chris Evans has said that he only did the Cap movie and the Avenger movie because RDJ told him it would be great for his career. And now he is signed for sequels, so as long as its in his contract he will do them. But he doesn't really want to.
So I am thinking these actors might only do whats in their contract, and since it seems Johansson doesn't have more cameos or other films besides a solo and more Avengers in her contract, I don't think the others would either.
So I don't think we will see Hawkeye pop up in a Cap movie, or Thor show up in an Iron Man film. I would love to see Banner show up in Tony's lab or Stark pop in to a Cap film as a friend he eats lunch with, but I don't think much of that will happen.
Also, for me personally, I don't want to see BW or Hawkeye in a Cap film. I sort of feel they would take some of the spotlight away from Cap. He needs his own solo film. SHIELD agents like Fury or Hill, or Carter, wouldn't because they are almost background characters, but Hawkeye and BW are now recognizable.
Let Cap have his own film.
Taskmaster wouldn't derail the avengers, I think. It does depend on where the movies go. But if they handle [BLACKOUT]Thanos[/BLACKOUT] in Avenger 2 or save him for Avengers 3 you still have one movie where they don't use him, unless its done in 2 and 3. But I think one of the Avengers films, with the original cast (they should keep it going past 3 films) should focus on the human element. How do the public see and treat the Avengers and what does their appearance mean for super criminals?
Just like Fury said to Iron Man and in the Avengers, more and more superheroes are popping up. But also more and more super villains. The Avengers movies should deal with the rise in super villains and what would happen if they banded together or if a terrorist organization used them. What if Hydra was hiding this whole time since Captain America's film? Maybe with the Avengers showing up their secret world domination plans are derailed so now they have to deal with the Avengers.