I had a feeling Zach didn't kill Cassie; it was just too convenient. It could have been the weepy boyfriend, considering Masuka's comment about the placement of the body suggesting it was someone she knew, but Cassie's bludgeoning was a little too similar to Norma Rivera's, so...
My next suspect is Quinn. He's surely the one who put Zach's blood under Cassie's fingernail. He had the opportunity, the motive (his obsession with nailing Zach), and the knowledge to pull it off; even Dexter was puzzled when he found out Zach didn't do it. But would he kill someone just so he could bust Zach for Norma Rivera's death? We'll see.
I think Dr. Vogel will be revealed as the "Brain Surgeon". First, her age makes her an unlikely suspect; perfect for a show like this. Second, she has intimate knowledge of, and familiarity with, all of the victims; this gives her opportunity. She could easily meet with them, lull them into a false sense of security, drug them, and dissect their brains while they're unconscious. Dumping the bodies might be difficult for her, but not impossible. Also, from a storytelling standpoint, there's a nice bit of closure if Dexter eliminates the person who actually made him who he is; maybe doing so will rid him of the Dark Passenger.
Hannah. I'm not sure how that's going to play out. If it weren't for Harrison, Dexter would probably run off with her. As is, in the end they'll probably fake her death and establish a new identity for her so that the Feds will stop looking for her. Or Vogel will kill her for "interfering" with Dexter. Again, we'll see.
