[pr0xyt0xin]

I dont know how your "defines the film" criteria works but Joss had no problem doing in Wash and Book in Serenity. They were definitely main characters. At that point, I think if he felt that it was integral to the story that he wouldve just as soon killed off Mal or River too. I never once got the feeling that he would spare a character just because they were the "main" character.

That being said, I think Cap is a little more likely to get the death sentence seeing as how his character will have exhausted his trilogy by that time and Hawkeye will still have plenty of potential Box Office take to rake in.