Well, Percy might die in a magical accident. Maybe he's trying to harness some extremely powerful and unstable magic, and it backfires on him fatally. He could also be the victim of a curse. A powerful sorcerer or a vengeful god could place a curse on him that slowly kills him. And in some fanfictions, Percy could be killed by a group of enemies who gang up on him when he's in a weakened state, like after a previous battle or when he's separated from his friends and allies.
One possible scenario could be that Percy dies heroically in a battle against dark wizards. Maybe he sacrifices himself to save his family or a group of innocent people. Another could be that he succumbs to a mysterious and incurable magical illness that has been plaguing the wizarding world. He might have been researching a way to stop it but in the end, it gets the better of him.
One possible scenario could be that Percy has faced so many traumas and losses in his battles against the monsters and gods' schemes that he simply feels exhausted and wants an end. For example, if he loses his close friends like Annabeth in a particularly brutal way, it might push him to the edge.
Maybe there's a powerful, ancient monster that Percy battles single - handedly to keep Artemis safe. The monster could be something like a Titan that has been awakened and is far too strong for Percy to defeat alone.
One possible plot development could be Percy and his friends facing a new, even more powerful enemy. Maybe an ancient primordial being that was previously sealed away has been awakened. For example, a new Titan or a primordial chaos entity. Another development could be Percy exploring his new powers or abilities that he might have gained after the events of Heroes of Olympus. He could be training other demigods as well, passing on his knowledge and skills.
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.
There are fanfictions where Percy and Annabeth are sent on a secret mission by the gods. In these stories, they often face challenges that test their relationship and their individual skills. For example, they might have to infiltrate an enemy stronghold where the magic is different from what they're used to. This forces them to rely on each other in new ways and deepens their bond.
One possible plot could be Percy being framed for a crime he didn't commit by the Olympians. Maybe they needed a scapegoat for something that went wrong in the mortal world that they didn't want to take the blame for. Another plot might involve Percy discovering a dark secret about the Olympians that they don't want him to reveal, so they turn on him to keep him silent. It could also be that a new powerful enemy has influenced the Olympians against Percy, perhaps by spreading false rumors or using some form of mind control on them.
A lesser - known but really interesting fanfiction is 'Annabeth's Quest for Wisdom'. This story follows Annabeth as she goes on a solo quest to gain more knowledge and wisdom. It shows a different side of her character, her determination and her thirst for knowledge in a very engaging way.
One interesting fanfiction could be about Jason and Percy teaming up for a super - difficult quest. They might use their unique powers in tandem, like Percy's water - related abilities and Jason's control over the wind. It could involve them facing a new, powerful monster that neither has encountered before.
A plot where the demigods have to team up with their mortal enemies, like some of the monsters, in order to face a greater threat that endangers both their worlds. This could lead to some really interesting character dynamics and moral dilemmas.