3. Hawkeye
How Would It Change The Franchise?Jeremy Renner's Hawkeye got the least development of any of the Avengers in the previous film, and Renner himself expressed disappointment with this, even parodying the character on SNL. But Whedon has said that Hawkeye has a bigger role in the sequel...could that bigger role be a heroic sacrifice? It wouldn't be the first timeHawkeye has died before in the comics and with Quicksilver and the Scarlet Witch joining the team, as well as a rumored Captain Marvel appearance at some point, the team may not have room for its resident archer. The death of a SHIELD agent on the Avengers' watch could also lead to a schism between Fury and the Avengers, which could potentially lead into a push for superhero registration, such as in the terrible Civil War crossover. I'd hate to see it, but Kevin Feige has indicated an interest in doing it as a film at some point.
Will It Happen?I think Hawkeye may be the strongest contender. He didn't really have much to do in the last film, Renner was disappointed in his role (publicly so), and Whedon writing a bigger part for him could mean Whedon's going to make everyone love the character right before he kills him off, which is something he's known for. Hawkeye's death may not have the story implications as some of these others, but I would say he's the front-runner as the most-likely candidate.