Well apparently someone on the show was going to be killed of for good, which apparently as been a spoiler for sometime, but I didn't read about it until after this episode aired. So if that's the case and their not going to go the easy way and just kill of Shane than Jeremy is really the only option.
The main three will never be killed off unless the actors want out, something I don't see happening since the show is a steady paycheck and their names outside of Ian aren't big enough to let them just jump into another project. Ian is dating Nina so I don't seem him wanting to leave anytime soon. Klaus, Rebecca, Tyler's werewolf friend, and Elijah are all heading to the spin-off. Bonnie who everyone would probably like to see dead won't ever die since she's the only witch. Matt is in the same boat as Bonnie being the only one of his kind (human) so I don't see him going anywhere. Tyler would have been a choice if they didn't have him leave town because killing him now would seems pointless. Caroline is the only other option, but her death would make Tyler pointless although it would give Klaus a reason to leave town for the spin-off.
I really not sure why, but Kat is the best character on this show. I find myself rooting for her regardless of the storyline. As for why she took the cure, my best guess would be to make some kind of deal with Klaus so she could stop running from him. Either a trade or to kill him. Also since the episode were Stefan 'interrogates' Kat back at the start of season two I've always saw those two as endgame. Kat and Stephan, Elena and Daemon just works and solves a lot of the relationship crap drama. Stephan does seem to be Kat's one weakness.
Although I do agree with you that at some point Damon will be the one who takes the cure. I'm not sure how that's going to happen, but he's the only one who went to search for the cure that was adiment about not taking it so of course he will be the one to get it. Also besides solving the sire bond storyline it will have the biggest impact on the character compared to everyone else. I can see that happening in the season finale which gives season five, at least at the start him dealing with being human, at first trying to convince others to turn him back which no one will do.