Joss Whedon is one of the masterminds behind what Marvel is doing with their cinematic universe. Not only is he developing stuff for the Avengers, but he's also helping in the process of all of the other films as well. In a recent interview with EW, the director talked about the villains, as well as killing off characters in the cinematic universe. When it comes to villains:
"Every movie is going to be a different villain. Loki’s awesome, and he is awesome in 'Thor 2,' but 'The Avengers' is a different thing. I worked with Tom more than any other actor [in the last film], because he was the only villain and there were six heroes. And he was as great as anyone I’ve ever worked with, and I get why people loved the character. But it’s an Avengers movie."
He then goes on to discuss killing off fan-favorite characters, and when he was asked if he thinks if any of the Marvel heroes could be killed off in the near future, he said,
“Um… maybe? But I’d have to have a really good reason, a really great sequence for [Marvel executives] to go, ‘We’ll cut off a potential franchise, that’s fine!’ They know, as any good studio does, 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?’”
So there's a chance that somewhere down the line some of our favorite characters could die. I seriously doubt it will happen, but you never know. If it makes sense for the universe and will help make the story arc better, then so be it!