Not sure if this thread has been made in the past. It probably has, but chapter 513 really showed how horribly the crew could be devastated by a strong pirate, so it may be worth further discussion.
Correct me if I'm wrong, but there has been two instances in the past where Oda has made the comment that one of the crew will die soon. Otherwise, I wouldn't really even consider it as a possibility. The first time was in reference to the burning of their ship - an emotional scene to be sure, but not what anyone was expecting. The second time was a joke about Brooke joining.
I know very few characters of importance die in One Piece, except in flashbacks. Oda is the type of manga-ka who loves his characters, and can't stand writing them out of the story without sending them off right (hence the mini-arcs). But, he has never shied away from very tragic events. And what could be more emotional than the death of a crewmate? So, what if one of the main crew actually does die? Who would it be? How would it affect them?