Well, these are just what I think could happen in The Sacrifice. I forgot what ep it's going to be, but I know there is an episode called that, so...

Theory 1: Katherine dies as the sacrifice. It'd be how she was eliminated, and I think might be a really interesting story line. It's a pretty simple theory.

Theory 2: Elena dies as the sacrifice. BUT, she has vamp blood in her system, and comes back to life. I could totally see Julie and Kevin go with this route, because it'd be right before the epic long break, and what better way to leave us than new born vamp Elena?

Theory 3: Both Kat and Elena die. Katherine as the sacrifice, and Elena by some accident or something. They also could leave us with new vamp Elena at the break, and it also removes Kat. Two birds, one stone.

Those are just my thoughts. I'd love feedback on what others think :)