100+ years post Battle of Hogwarts, minimum. I get a real kick out of reading these.

A few good ones I've encountered:

The 312th Edition:
http://www.siye.co.uk/siye/viewstory.php?sid=12053

Report on the Excavations of Sites HH-87 and HH-88:
http://www.siye.co.uk/siye/viewstory.php?sid=126720

Now and Forever:
https://archiveofourown.org/works/3904648

The more it reads like a history book, the better.

 

One of One:
https://archiveofourown.org/works/23805088
200+ years. crossover with Star Trek: Voyager.

Avatar:
https://www.fanfiction.net/s/9622291/1/Avatar
Avatar(the movie) crossover.

Gods Among Us:
https://www.fanfiction.net/s/7725354/1/Gods-Among-Us
Whatever time frame Battlestar Gallactica is set in.

Invincible:
https://www.fanfiction.net/s/13161929/1/Invincible
Another BSG Crossover.

That last one technically doesn't count, but I feel like spaceships & sentient AIs makes it count.

 

Any non-crossovers?