I personally think it's going to be Carol. Poor Daryl lost his brother in the first season and gets pissed off, in 2nd season he started to care for Sophia but loses her so he gets pissed again. Now it's the 3rd season and he's growing fond of Carol, but as we all know, we're all sadistic fucks so the director will take away Carol to piss off Daryl again.
Now to be serious. I think this is actually feasible. Meryl or however you spell his name is coming back. Daryl is obviously turning into something Meryl will not approve of when they eventually meet. Add a shock of having yet another person he starts to care for being killed, Daryl's personality will spiral out of control again, with Meryl being there to add fuel to the flame. I'm a huge fan of Daryl, and though I want to see him happy, I also enjoy seeing his personality being challenged / changed by the ever changing environment. Either that or I'm also one sadistic fuck =)