Ok, I got the general feel that Hogwarts would not be a functioning institution in Book 7. I can't recall the exact lines, but it was at the end, after Dumbledore died, and Lupin I believe said that no one would want to send their children there.
SO, going off of that assumption (and we know what happens when you assume, but oh well) then we can assume Voldemort will die (which is pretty much a given anyway, it's just wether Harry will die or not that's up in the air), because we know someone will go back to be a teacher at Hogwarts (one of JKR's interviews) - and that won't happen until he (Voldemort) is out of the picture.
Not really a theory, just a general train of thought.
And oh, has anyone given thought to maybe it's Harry who is supposed to be a teacher? DADA of course. Think about it, first with the whole DA thing, and him being quite good at it, and later Fred and George commenting on it. And then, well, in order to prove Voldemort wrong, he could hold the position longer than a year. But yea, what do you do after you've destroyed the worst Dark Wizard ever? I doubt he has any ambition to become Minister of Magic, and he wants to be an Auror because, well, it's really the only thing he's been offered. And that, my friends, is something bordering on a theory - I don't know how firmly I buy into it, and it has no solid evidence (though maybe I'll supply some in the future), but yea... food for thought.