I think if it was going to be someone big, they wouldn't have done a cliffhanger - Daryl or Glenn would have a big enough impact that the cliffhanger was unnecessary.
I'm still thinking Sasha or Abraham.
Daryl would irritate lots of people. The mid-season finale and the first few episodes of this half of the season had us thinking Glenn was dead (although most of us had cracked it, we were supposed to think he was a goner).
I can't decide on Michonne. It doesn't feel like she's been around enough recently for it to follow but Rick's lovers do tend to meet terrible ends!