1) Everyone dies!
Wow! What a way to end the book, the entire magical word obliterated as the muggle prime minister reveals the existence of a "Magic Nuke" designed only to harm magical beings allowing muggles to continue with normal life as if wizards weren't even real.
Probability: Less then 0.1%
2) Harry dies!
This may come as a shock to younger readers, there was a time when there was no such thing as Hype, you'd watch an upcoming episode of Neighbours and BANG! Joe Mangle dies!! No warning at all leaving you shocked and shaken.
It was good, it was unexpected and it made the world a more edgy place.
Then came the 90's and the era of hype began :(
The chances of Harry surviving are very slim, but it's entirely possible he could survive, prehaps becoming a muggle would be an "out" for him.
Probability of harry dying: 80%
3) Voldermort lives!
Voldermort, the big bad of the wizarding world could survive, killing Harry and the rest of the order before again the muggle prime minister sends the SAS, few wizards will survive a sniper round fired from a mile away, but it's not Harry or any of his mates that kill him.
Of course Voldermort as evil overlord of a wizard society sounds well and good as well, but how sustainable is a society comprised of nothing but wizards?
Probability of Voldermort surviving: 5%
4) Held over for a new series of books
So no more Harry Potter books, but how about books covering Hermonine, Ron, Ginny even, a undisclosed ending, one where there is no resolution and you are left crying out for something to fill the void would be perfectly British.