Abrams. Been saying this for months. Favreau would be great but that's near impossible.
My idea for all of them to play apart in their own meaty roles goes like this:
Thor - Being that Loki is the main villain, Loki returns to Asgard and takes it over, banishing Thor from Asgard and never returning. He sends him back to Earth. Loki doesn't know about the Avengers. But Thor remembers what he told Fury about him working with them if they ever needed them (per the Thor articles) so he joins the team to save his kingdom and father and get Loki. So people question if he is really there to help or to help his kingdom.
Stark - We've discussed this, but Stark is disapointed by Cap He expects some great noble warrior and is disapointed. He sets his standards too high. He feels he needs to lead the team because Cap is adjusting so he feels that Cap can't do it. Which is where they butt heads. On leadership. Cap thinking his ways are good, but Stark doesn't feel it can work. He just doubts him because times have changed and doesn't believe he can follow through. It's no longer WWII. But eventually, Stark has to realize that Cap IS the true leader of the Avengers. Maybe Tony and Rogers have to do something together in the third act and work together. I see this like the Kirk/Spock relartionship in ST. Them not liking eachother at first but then they devlop a good camraderie. This is why I think Abrams can handle this.
Captain America - like I said, he is in a new time and century. No longer WWII. Enemies have changed, and so have heroes. Being that he was the only hero. But he has a hard time nderstanding and adjustiong to these new times, a point where Stark thinks he isnt' fit for the leader. But Cap has to realize he must step up and be the leader. It was never going to be the second he was unfrozen. It takes time to know what to do.
Banner - Banner is still hiding out in the mountains. Ross visits him by himself. He tells him they will leave Banner alone if he agrees to join a special team. Ross has made a deal with SHIELD to use him. Banner is reluctant and just wants to see Betty. He doesn't trust himself. Ross tells him face to face if he doesn't accept, they will come after him with force, eventually Ross will come after him, and he will be on the run again. Banner is the last member to recruit. SHIELD shows up when Bruce plans to leave the mountains. Rogers, Thor and Stark show up. Rogers wants to talk and believes Banner and the Hulk can be controlled and trusted. Stark is of course reluctant about it all. Which leads into the trust theme. When Banner runs, they anger him somehow. After Cap tries to subdue him, he fails, further questioning himself as a hero and suppose leader. Stark in his own ideals also fails. Thor eventually manages to defeat Hulk. Thy take him back to wherever and throw Banner in a cell. Which leads to Cap and Stark bashing heads about how to use him. Eventually he comes through in the end and they can trust Banner. He also accepts being on the team because he is wanted. At the end, Banner eventually sees Betty for the first time since TIH.
The overall theme is camraderie and trust. Trust in another and even in yourself. These heroes aren't all perfect. It addresses what it means to be a hero. It could come off as cheesy, but they need to get it right. These people have their share of problems, but hopefully they don't come off as *****ebags.