The Last Olympian happened, but Annabeth and Percy never got together, and Percy accepted godhood, becoming the 15th Olympian (Hades and Hestia have their thrones returned to them). Percy, forced to be known as Perseus, is the god of the tides, and the minor god of the moon and the ocean.
The war against Gaea never happened.
It has been roughly a century since the end of the second Titan War, and Perseus becoming a god.
Disclaimer for whole story: I OWN NOTHING!!!!