As you probably know somebody will die in Thursday's mid-season finale. That character is somebody we've known since season one. Let's assume that Elena, Damon and Stefan are safe.
That leaves: Tyler, Caroline, Matt, Bonnie, Jeremy, Sheriff Forbes and Mayor Carol Lockwood.
This is my theory: Tyler and Caroline are probably safe as their love triangle story just started. Bonnie is also safe as her father arrives in 4x10 to tell her to stop seeing Shane. Shane needs Jeremy for his plan, so he's also safe. That leaves Matt (who is the only main cast human being) Liz and Carol. Since Tyler is going against Klaus, maybe Klaus will kill his mother as a revenge. (We also know that there will be a lot of casualities (read "dead hybrids") in the episode, so maybe Klaus wants to avenge them.