Spoilers ahead and all that.
I'm thinking the grandparents are the most likely outcome, but it will be sort of even sadder if both Rita, Astor and Cody disappear from the show at the same time. The single dad-aspect will definitely be interesting. I read an article earlier today where someone suggested that Harrison might end up in a foster home, but that makes no sense to me. For everyone else in Dexter-verse, Dexter is a seemingly normal and upstanding guy (excluding the fake drug problems) who should be fully capable of taking care of his own son, and I also don't think he would let someone else take care of Harrison after having basically the same thing that happened to himself happen to his son. So yeah, I'm looking forward to seeing how they deal with the single dad vs. serial killer-thing. Do you think they will jump ahead in time a couple of years or something, to make Harrison a bit older? I've seen someone mention that as a possibility, and it wouldn't surprise me, but I don't quite know what I want.