One possible plotline could be Percy being betrayed by the gods and thus deciding to take on Olympus. Maybe he discovers some dark secrets about the gods' rule that make him believe destruction is the only way to bring true justice.
Perhaps in the fanfiction, Percy is trying to reform Olympus but his attempts are constantly thwarted by the corrupt elements within the pantheon. Frustrated, he resorts to more extreme measures and decides to destroy it in order to rebuild it from the ground up in a more just and equal way. His journey would then involve gathering allies who believe in his vision and fighting against those who oppose any change to the status quo.
In some 'Percy Jackson Raised on Olympus' fanfictions, there could be a plotline about Percy uncovering long - hidden secrets of Olympus. For example, he might discover a forgotten prophecy that has implications for the entire pantheon. Also, his interactions with the nymphs and other mythical creatures in the Olympian realm can make for an exciting story. Maybe he has to protect them from a new threat that is emerging within the boundaries of Olympus itself.
One common plotline is Percy finding out that the Olympians have been using him all along. For example, they might have sent him on dangerous quests not just to save the world but also to gain more power for themselves. Another could be Percy being betrayed by a close friend or ally who is working for the Olympians' self - serving plans. Maybe they set him up to take the fall for something he didn't do.
Grover could also be a major character. As Percy's best friend, he would probably be involved in trying to stop Percy if he thinks Percy has gone too far. Or, if he sees the corruption in Olympus, he may side with Percy. Grover's connection to nature means he could play an important role in the events that unfold, like using nature spirits to either protect Olympus or aid Percy depending on his stance.
There could also be some ancient monsters or spirits that are tied to Olympus' existence. They might rise up to defend it because their own survival is linked to the continued existence of Olympus. These could be powerful beings that Percy has never faced before, adding an extra layer of danger to his quest to destroy Olympus.
I'm not entirely sure specifically as it's a fanfic. But generally, it might involve Percy having some extraordinary power or facing a situation that forces him to destroy Olympus. It could also be a really creative take on the existing Percy Jackson universe, where the author wants to explore a different and more radical storyline compared to the original.
A great fanfiction idea is to have the heroes of Olympus time - travel. They could go back to the time of the ancient Greek heroes and interact with them. Maybe Percy meets Theseus and they have to compete in some sort of challenge. Or the story could focus on a character like Nico di Angelo and his journey to discover more about his powers in a place that is off - limits to most demigods, like the Underworld's deepest secrets. There could also be a fanfic where the demigods have to deal with a curse that affects their powers in strange ways.
Maybe there's a plot where the demigods are trying to improve the living conditions on Olympus. Since it's a place full of powerful beings, it might be in a bit of chaos. So they could be working on things like building better homes or creating a more organized system for the lesser - known gods.
One idea could be Percy having to face a new, unknown monster that is immune to water. He'd have to rely on his other skills and the help of his friends in a completely different way.
In a Percy Jackson Warrior of Olympus fanfiction, a great plot could involve Percy being sent into the future of the demigod world. He sees the consequences of the current actions of the gods and demigods. For example, he discovers that a war between the different pantheons has led to the destruction of many demigod camps. So, he has to go back in time and try to change the events that lead to this future. This would involve him having to convince not only his friends but also the gods to change their ways, which would be no easy feat as the gods are often stubborn. He might also meet some of his descendants in the future, and they could help him understand what exactly went wrong.
One idea could be Percy discovers a new, hidden power within himself that is linked to an ancient prophecy. Maybe he can control the elements in a way no demigod has before.