How about Voldemort atoning for all his sins in the end? Can this be possible under the circumstances?
To which I'd say: that would probably be an ending no one expected, but not altogether unsatisfying. It all depends on how Rowling would pull it off.
And that reminds me of one of the entries in a contest held some while back by the Guardian, Dumbledore's death in the style of GK Chesterton, written by Zander Nyrond.
Here's the entry:
The study of Albus Dumbledore, headmaster of Hogwarts School of Witchcraft and Wizardry, was at the best of times a crowded room. It was even more crowded at the moment, being enriched by the presence of Cornelius Fudge, the Minister for Magic, Kingsley Shacklebolt of the Aurors and a dumpy little man in a long black robe and shovel hat.
Strangely enough, though, the man with most right to be there was absent; and the man who should have been triumphantly in charge of the situation was handcuffed and glowering in the grasp of the Auror.
"But how did you know, Father Brown?" cried Mr Shacklebolt.
The little priest blinked. "Oh, well, you know," he said shyly, "there was the medal. Why on earth would this Voldemort go out of his way to melt Professor Dumbledore's Order of Merlin? What was it to him? Merely a bauble. But to Fudge, don't you see, it was a symbol of his hatred of Dumbledore. He hated him," said Father Brown earnestly, "for the unforgivable sin of being right.
"But I knew before that, of course," he went on. "I had a long talk with this man you call Voldemort last night, when I found him up to some devilry in my churchyard. He's a wretched soul, of course - this magic of yours does so twist things out of their true form and purpose - but I fancy I found some good in him."
He got up, cast about helplessly for his umbrella, found it under the table and went to the door. There he turned, and the twinkling lights of the room were reflected in his little round spectacles like the stars of heaven.
"Your wizarding world could really do with some priests," he said, smiling. "It really could, you know."