Note: Iam completly unspoiled, religiously so, and I do not want guesses confirmed or denied by those who are spoiled, please!
This prediction is based upon the following givens:
Buffy can’t die. It would just be to lame: I mean, she dies again!? No poignacy there.
Buffy can’t continue to be the slayer. I demand an ending, since we can’t just believe that Buffy toils on as the slayer well into middle age.
Buffy can’t lose her slayer powers. It’s already too late in the season fot that, since if she were going to lose her slayer powers, we’d really need at least half a season to see her come to terms with it and pass on the torch.
It can’t end too neatly. The central theme in Buffy–the key to the whole show–is that there is always a gap between whatyou want and what you must do.
She can’t wake up in a mental institute: I’d break my television, and I can’t afford to replace it.
Therefore, the only possible ending for Buffy is:
She ends the series as a vampire with a soul.
She has to: it fits all the criteria–she’d stop aging, she’d keep her powers, she’d stop being the slayer.
Furthermore, it would be the ultimate “duty over pleasure” thing. She’d be doomed to live FOREVER fighting evil, never resting, never getting to die and go back to heaven (which she knows rocks), having to watch her friends and family grow old and die.
To contrast, I suspect that Spike will have at least a chance to become human again–I don’t know if he will accept the change or stay a vampire with Buffy.
A less complex but also acceptable end would be if Buffy were thrown back in time and became the first slayer.