2. Clint Barton's "Death"
It's not enough for one main character who can't actually die in conventional terms - to die, nor will the death of Jim Rhodes be significant enough to blow the Avengers world apart. There's some speculation that Pepper Potts might also be up for the chop, since it seems likely that Tony Stark is going to be a significantly broken man by the end of the film (enough to decide to take on Hulk at least), but that might just be over-egging it slightly (especially since she's already "died" once). The more fitting assumption is that one of the Avengers themselves dies, and the only likely options are Black Widow or Hawkeye because of the narrative responsibilities of everyone else (or the fact that they've already "died" in the cases of Nick Fury and Thor). Because Barton is getting more focus, he leaps to the top of the list of likely candidates, and he has died in the comics fairly spectacularly in the past. And of course you have to consider who the least missed character would be, and the answer is unfortunately clear. An Avenger death might also account for the climactic battle between two of the Avengers: clearly something devastating has to lead to Hulk being reclassified as a threat...