I'm going to go with Spike. I'll illustrate the reasons I think this might be a possibility on the predictions thread, when I get a chance..
As for the First, well.. There's nothing saying the First can't be used. I mean, here we are after season 4, a season that used a God, much like Buffy's 5th season. The Big Bad concepts ARE recycled, on occasion. Additionally, the First always seemed much more concerned with 'The Vampire with a Soul' than the slayer, and although the concern re: spike was well-validated in the final episode, it's hardly wrapped up. The First tends to take the long view, and is still around, so why was it more concerned about Angel and Spike than the Slayer, who played an equal part in foiling it's plans? I think there is much more to be done with the First.