One possibility could be that after facing too many dark magic encounters, Harry's mind starts to break. Maybe the constant threat from Voldemort and the losses he endures, like the death of his friends, push him over the edge. He could start to have hallucinations and lose his ability to distinguish between reality and dark magic-induced illusions.
In the 'harry potter goes insane fanfiction', it could be that a hidden family curse surfaces in Harry. This curse could be related to his ancestors' involvement with dark magic. As the curse takes hold, Harry begins to lose his sanity. His once noble and brave personality is replaced by a paranoid and unpredictable one. He may even start to believe that his friends are his enemies and try to isolate himself from the wizarding world he once loved.
Perhaps in this fanfiction, a powerful and evil spell backfires on Harry. For example, if he tries to use a spell he doesn't fully understand to defeat a new dark wizard. This spell could start to corrupt his mind, making him go insane. He might then start acting erratically, attacking his friends by mistake as he is no longer in control of his thoughts.
It could be a new dark wizard who has the power to manipulate minds. This wizard might target Harry specifically, driving him insane as part of a larger plan to take over the wizarding world. Since Harry is seen as a hero, if he goes insane, it would create chaos and make it easier for the antagonist to achieve his goals.
One possible plot could be that Harry's insanity is caused by a cursed object. His friends then have to find a way to break the curse while also dealing with his erratic behavior. For example, Harry might accidentally reveal secrets to the wrong people because of his insanity.
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.
Maybe it could be due to the loss of his friends. For example, if Sasuke died in a battle and Naruto couldn't save him, the grief might push him over the edge and into insanity.
Well, perhaps Harry Potter goes to Dalton and discovers a whole new world of magic there. Maybe he meets new friends and has to face different challenges compared to Hogwarts.
In this fanfiction, it could be that Harry Potter has a vision of a future where Voldemort has completely taken over the wizarding world and killed all his loved ones. This drives him into a state of madness and he goes on a rampage. He travels back in time using some ancient magic he discovers, and starts to take out all the Death Eaters one by one, not caring about the consequences, as he believes it's the only way to change the future.
In a 'Harry Potter goes to jail' fanfiction, perhaps Harry breaks a new law that the Ministry of Magic has created to maintain control. For example, if he tries to teach magic to non - magic people in an attempt to prepare them for a potential dark wizard threat. The Ministry, fearing chaos, throws him in jail. Inside, he meets other wrongly - imprisoned wizards and they plan an escape together while also fighting against the unjust system.
Maybe it's about Harry Potter being framed for using dark magic in a way that violates the wizarding laws. As a result, he has to go to this 'juvie' place. In there, he discovers that there are other kids who have also been wrongly imprisoned. He then uses his skills and the loyalty of his friends (who might try to break him out or help him legally) to fight against the unjust system.
In this fanfiction, perhaps Harry goes to Azkaban to rescue someone. It could be a fellow wizard who has been unjustly imprisoned. He might have to use his skills and some new-found magic to navigate the dangerous and dreary corridors of Azkaban, all while avoiding the Dementors.
One possible plot could be Harry struggling to fit in at Harvard. He might be the only wizard there, so he has to hide his magical abilities while also trying to keep up with the intense academic curriculum. For example, he could use his quick thinking skills from his Hogwarts days to solve complex problems in his Harvard classes.