Finally! I agree that Yates didn't seem to be the Brain Surgeon; I would have been so disappointed had he not made a return.

TheoriesVogel seems to be a possible candidate herself, and I'm still assuming she at least knows or did something in her past that she's not telling Dexter. I still don't think she's the killer yet (though maybe she's working with the killer...).

Really random idea: what about Cassie's (now former) boyfriend, or even Elway? They would make an interesting parallel to the Ice Truck Killer: dating (or trying to) someone near Dexter. Neither seems to have much interest in Dexter as yet, so at this point this is entirely out in left field, baseless speculation. But it's potentially noteworthy that Cassie's bf did see Dexter with Zach.