Re: Who would you let Joss Whedon kill in Avengers 2?
If they should kill someone that really created a big impact it would be through killing one of the main four. Out of those I think Hulk is the best option.
First of all it would lend the killer huge credibility since Hulk is extremely tough and durable. To have Thanos not being truly beaten in the second movie and having him wreak such havoc as to have Hulk die could be somewhat close to being the Empire Strikes Back of this series. Secondly Hulk doesn't seem to get any more movies, and perhaps they've deemed the he works best in short stints.
Iron Man would be the other option since you can still have Iron Man without Tony Stark. That said, Stark probably carries the movies most of all characters overall and is really the base for the popularity of the MCU so that doesn't seem as wise or as likely.
Captain America dying wouldn't be as special since he's the weakest member of the four and his humanity is pretty important for the group. Thor could give the same credibility to a villain as he's a powerhouse just like Hulk, but he's already died several times in the comics and it feels done, plus that he's of course the second biggest franchise after Iron Man.
So do I want Hulk to die? Of course not. But if a character that I want to see die, or that I don't really care about dies, then the death doesn't do it's job.