It could be an ancient deity that was banished long ago. Maybe a primordial god or goddess who was considered too powerful or too dangerous for the other Olympians. For example, it could be Erebus, the god of darkness.
One popular theme could be Percy's adventures within the Olympic realm. Maybe he discovers new powers or meets unique Olympic deities. Another theme might be his relationships with other characters in this fanfiction's take on the Olympian world, like forming alliances or facing rivalries.
Well, Percy is obviously one of the main characters since the title mentions him. Other likely characters could be the Olympian gods and goddesses. For example, Zeus, Poseidon, and Athena are important figures in Greek mythology and would probably play significant roles in a story set in the Olympian world.
Well, perhaps Percy has to go through a series of trials. He might need to prove his worthiness to the Olympians. Maybe he has to battle some mythical creatures that are threatening the Olympian realm. And in the end, he gains the recognition and powers of an Olympian.
Well, it could be about Percy facing a new and powerful threat that even the Olympians can't handle easily. Maybe he discovers a hidden 13th Olympian who has been locked away for centuries and now is causing chaos.
Well, you could write a fanfiction where Percy loses his memory and has to rediscover his powers and his place among the demigods. Or perhaps a story about how the minor gods in the Percy Jackson universe form their own alliance because they feel overlooked by the Olympians. You could also create a fanfic that focuses on Annabeth's journey if she was separated from Percy and had to fend for herself in a dangerous situation.
In this fanfiction, it could be that Percy is chosen for a special task by the existing Olympians. They see his potential not just as a hero but as someone who could join their ranks. So he gets trained in Olympian magic and lore, which is very different from what he's used to. He has to learn how to control new powers and also deal with the politics among the Olympians.
The Titans could be the main antagonists. They might oppose Percy's ascension to Olympian status because they see it as a threat to their own plans of overthrowing the Olympians.
Fanfiction based on 'Percy Jackson: The Last Olympian' can be quite diverse. Some might focus on the relationships between the characters, like Percy and Annabeth's relationship evolving in new ways. Others could explore the idea of new threats emerging in the demigod world. It's also a great way for fans to connect with each other, sharing their ideas and interpretations of the story.