I've been thinking about what would be the most unexpected thing to do with the final Cylon and what occurred to me is that all the speculation is focused on the living. So why couldn't the final Cylon be someone that has died in the Series to date that they happen to come across on some planet or base in the final episode of the first half of the last series. And then we get some back story on them as the start of the 2nd half of the series.
After all we should be able to presume that the final five can resurrect just like the other 7even flesh jobs. And so this final member of the final five may have died and gone to some place where they were resurrected (where a place had been prepared for them as the Testament would say), and we've had a small bit of foreshadowing in that Lee Adama asked his father what if it had been his brother that had returned rather than Kara. So my wild guess is that the final Cylon could be the other younger Adama boy Zak. Or more probably but still on the same theme another character who has been killed in the series so far, like the President's old PA - Billy Keikeya.