As much as I'd love it, as great as i think it would be for the plot and for Luffy's growth and development, I just don't see it happening, at least not in this arc. It's possible he gets captured and killed off screen, but even that is unlikely. Oda has a thing against killing characters unless it's extremely important to the story, and I just don't see it. If Oda's going to kill someone, it would probably be something like Blackbeard killing Shanks or Akainu killing Dragon or something monumental like that. Oda seems to save death for when it's absolutely necessary, and I don't think killing Doflamingo is absolutely necessary, so I don't see it happen, though I'd be thrilled if it did.