Really Hawkeye is the only one of the Avengers you can kill off.
The friendship between the big three should, and hopefully will, become the core of the movies. You do want to build stakes for the audience and keep the threat tangible but you don't want to tear out the series' still-beating heart. Otherwise you're left with just the thing's shell.
Learning to deal with and channel the monster inside is the entire point of Hulk's character, in many ways his death would be a reprieve instead of a sacrifice and/or shock. Thematically it sends the wrong message. And Black Widow is the token female, and in some ways the audience surrogate.
You can kill off Fury or Hill too but it's not as strong as killing off one of their own, a member of their little "family".
Plus an awesome heroic death is the perfect thing to take Hawkeye from a fun side character and turn him into a hugely memorable character in his own right, for fans and non-fans alike.