Harry Potter and the Nightmares of Futures Past is a redo fanfiction set in the Harry Potter books. The story was written by Matthew Schocke who has stated in a podcast interview that he felt that a central factor in the plot of the original books was the knowledge that Harry did not have in order to make informed decisions. He therefore wrote the story to explore what would happen if Harry did in fact have actual foreknowledge of events.
The premise of Nightmares of Futures Past is that the war between Harry Potter and Voldemort lasted for more than a decade and that although victorious, Harry was left friendless and alone, all his friends having fallen during the conflict. As a result he attempts a daring experiment to project himself back in time to merge with his childhood self and hopefully defeat Voldemort without the terrible costs of his own history.
Sadly, it was never finished.
S'TarKan (Matthew Schocke) has published the 40th chapter, and is going to continue the fanfiction.
Here's a quote from the author:
Yes, I am back on a semi-regular writing schedule again. I'm off to work on Team 8 now. Yes, you could consider this a cliffhanger… but not when you think about it. Enjoy!