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.
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 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.
A possible plotline could be Naruto using his wall - passing ability to infiltrate enemy strongholds undetected. He could gather crucial intelligence or set up surprise attacks. For example, he might slip through the walls of Orochimaru's lair to rescue someone or find out his latest evil plans.
A possible plotline could be Percy being assigned to a top - secret mission by Shield. He has to use his unique skills to infiltrate an enemy organization. Another plotline might be that Percy discovers a conspiracy within Shield itself and has to decide who to trust while trying to expose it.
Perhaps Medusa has some crucial information that Percy needs to save the world, and in order to get that information, he has to protect her from other threats. So he embarks on a journey to keep her safe.
One possible plotline could be that Percy has been kidnapped by some mysterious enemy. Maybe it's a new villain who has a vendetta against him. His friends then start a search mission to find him, facing various challenges along the way.
One possible plotline could be Percy struggling to adapt to his color blindness in a world where color is important. For example, in a magical setting, colors might be associated with different powers or elements, and his color blindness could be a hindrance that he has to overcome.
One possible plotline could be that Percy has been wronged by Artemis in some way, perhaps she made a decision that put Percy or his friends in danger, and now he's out to punish her by thwarting her plans. For example, if Artemis made a deal with a villain that backfired on Percy's team, Percy might seek revenge.
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.