One possible reason could be that Percy feels betrayed by the Olympians first. Maybe he discovers some dark secrets about them that make him lose faith. For example, if he finds out that they have been using demigods unfairly in their battles or politics.
In this fanfiction, perhaps Percy is influenced by some dark force. This force convinces him that the Olympians are corrupt and need to be overthrown. He then betrays them by stealing important artifacts from Olympus that are crucial for its protection. He might also try to turn the demigods against the Olympians.
The Olympians themselves could be the main antagonists in a sense. Since Percy is betraying them, they would be the ones trying to stop him. Zeus, with his power and pride, would likely be at the forefront, seeing Percy's betrayal as a direct threat to his authority.
Well, usually it starts with Percy having some sort of conflict with the Olympians. Then he starts to make plans in secret. He might gather a group of supporters, like other disgruntled demigods or even some mythical creatures who also have a grudge against Olympus.
One possible reason could be to create a tragic story. Writers might want to show how the world of the story would be affected by the loss of such a central character like Percy.
Perhaps the story involves Percy being framed as an assassin of Olympus. He then has to clear his name while dealing with the real assassin who is causing chaos among the gods and demigods. The plot might also include Percy's journey to discover who set him up and why.
In such a fanfiction, perhaps Percy could use his unique powers in new and unexpected ways. He might gain the support of some unexpected allies among the demigods or even some minor gods who are tired of the Olympian regime. This could lead to a large - scale battle where Percy's strategic thinking and combat skills are put to the ultimate test against the might of Olympus.
He could be involved in some really cool quests. Like finding some ancient artifact that's hidden deep within Olympus. He'd probably make new friends among the minor gods and goddesses. And of course, there would be the constant threat of some ancient evil trying to take over Olympus, and he'd be right in the middle of the fight to defend it.
Annabeth could definitely be one of his main allies. Her intelligence and strategic planning would be invaluable in a fight against Olympus. Also, Grover might be there, using his skills as a satyr to gather information and support from other nature - related beings. Some of the less - well - known demigods from Camp Half - Blood could also join in, seeing Percy as a chance for a new order.
His friends from Camp Half - Blood would be his allies for sure. Annabeth, with her strategic mind, would be invaluable. Grover, the satyr, could help Percy communicate with nature spirits which could be useful. But also, some of the minor gods who feel left out by the major Olympians might side with Percy. For example, Hecate could use her magic to assist Percy, seeing him as a chance for a new order in the Olympian world. There's also Tyson, Percy's cyclops half - brother, who would be a strong and loyal ally.
Maybe he accidentally breaks some ancient and unforgivable rule. For example, he could have uncovered a secret that the gods wanted to keep hidden and in their attempt to silence him, they banish him. In fanfiction, the writers often play with the idea of Percy's curiosity getting him into big trouble with the Olympians.