As long as they have their own franchise, it's highly unlikely that filmmaker Joss Whedon would kill off one of Marvel's major superhero characters in the upcoming "The Avengers" sequel.
Then again, Whedon's work has often featured surprise deaths as a means to move story and character development forward. This was notably seen in the first "Avengers" regarding the apparent death of Agent Coulson.
Whedon has also stated he plans to "rotate members" of the Avengers, so we'll see some heroes come and go throughout. so it begs the question, would he be willing to kill off some of these characters. He tells EW:
"Maybe, but I'd have to have a really good reason, a really great sequence for [Marvel executives] to go, 'that's fine!' They know that without some stakes, some real danger, how involved can we get?
We don't just rule it out across the board, but neither is the mission statement 'Who can we kill?' We try to build the story organically and go, 'How hard can we make it on these people?' You go to the movies to see people you love suffer."