In some 'percy goes insane fanfiction', it might be that Percy discovers a dark secret about his family or his own past that he can't handle. This knowledge could shatter his sense of self and sanity. For example, if he found out that his powers were actually a curse passed down through generations in a more sinister way than he thought, it could be too much for him to bear.
One possible plotline could be Percy having a series of traumatic experiences that gradually drive him insane. Maybe he loses his friends one by one in tragic ways, and the cumulative grief and guilt push him over the edge. Another could be that he is exposed to some dark magic that slowly corrupts his mind, making him lose his grip on reality.
One possible plotline could be that Naruto is pushed to the brink during a fight against a powerful and sadistic enemy. Maybe this enemy has been targeting his friends one by one, causing Naruto to gradually lose his sanity as he tries to protect them. He might start using extremely violent and uncharacteristic jutsu, taking risks that he normally wouldn't. Another plotline could be that Naruto is under the influence of a dark genjutsu during a fight. This genjutsu twists his perception of reality, making him believe that his friends are actually enemies, and he goes insane trying to fight them off.
One possible plot could be that Naruto is constantly ostracized by the villagers even after all he has done for them. The years of being treated as an outcast and having his good intentions misconstrued finally push him over the edge. He starts to see the villagers as the real enemy and decides to use his power not for their protection but to make them feel the pain he has endured.
Obviously, Percy would be the central character. His friends like Annabeth, Grover, and other demigods from Camp Half - Blood might also be important. They could be trying to help Percy or might be affected by his insanity. For instance, Annabeth might be desperately searching for a way to bring Percy back to his senses.
One possible plotline could be that Percy has been betrayed by his closest friend. Maybe this friend revealed a big secret of Percy's, and now Percy feels isolated and heartbroken. He might start to question his relationships and his place in the world.
One possible plotline could be Percy's friends trying to break him out. They might have to overcome various obstacles like guards, traps, and security systems within the prison.
One possible plotline could be Percy struggling to fit in at high school while also hiding his demigod powers. Maybe he accidentally uses his powers in front of his classmates during a sports event and has to come up with a creative excuse.
Perhaps Percy gets involved in a special mission at Camp Jupiter. Like he has to protect a powerful ancient artifact within the camp from an outside threat. This could involve a lot of action and strategy as he navigates the Roman camp's politics and military structure.
Well, perhaps in the fanfiction, Annabeth betrays Percy's trust in a relationship way. She could start seeing someone else and use Percy's secrets against him to make herself look better in front of the new person. Or she might side with a group that Percy is against, believing their lies and false promises over Percy's warnings. This could lead to Percy feeling completely alone and having to find new allies.
Another plotline could involve Percy making new friends in the navy. These friendships could be tested during dangerous missions or when faced with internal conflicts within the naval unit.
Another plotline could be that Percy's reincarnation is part of a grand plan. There could be a powerful being or a prophecy that dictates his reincarnation. When he is reborn, he is fated to either save or destroy this new world he is in. Along the way, he meets friends from his past life who have also been reincarnated, but they don't recognize each other at first, and they have to go through a series of adventures together to remember who they really are.