I don't see Wash dying until the end of Firefly's run, if it had lasted to five or six seasons. (Maybe because I really want to see a little Wash or Zoe running around the ship...?)

I think Book, though, had such great potential -- his past as a former high-ranking official had been hinted at during the show, but his backstory was never fully revealed. I could see him leaving the crew because of sudden plot surprises because of his past and be a smaller but still interesting character (like Dru was in Buffy/Angel).