It could be that Naruto discovers some dark secrets about his past, like his parents were actually killed by the very people he trusted in the village. This revelation is so shocking that it shatters his mental state. He becomes a completely different person, losing his sense of right and wrong, and his sanity along with it.
Maybe Orochimaru manages to get his hands on Naruto and subjects him to some dark and twisted experiments. These experiments could mess with Naruto's mind, causing him to lose his sanity. He could start seeing illusions all the time, believing his friends are his enemies. As a result, he'd be in a constant state of confusion and aggression, and his once - positive attitude towards life and ninja - hood would be completely gone.
One possible plot could be that Naruto's loss of sanity causes him to have a distorted view of the Fem Kyuubi. Maybe he sees her as both a threat and a source of power he can't control. This could lead to a conflict within himself as he battles his inner demons while also interacting with the Fem Kyuubi in unexpected ways. For example, instead of the usual relationship of cooperation or taming, he might try to completely dominate her, which would be a big shift from the normal Naruto we know.
Perhaps an unknown and powerful dark ninja. This ninja could be from a long - forgotten clan with dark powers. They might have a vendetta against the Leaf Village and see Naruto as the key to their revenge. So they use their dark arts to make Naruto lose his sanity and turn him against his own village.
It could be Kaguya. If Kaguya were to return and use her powerful abilities to drive Naruto mad, perhaps by invading his mind with dark thoughts and memories. She has the power to warp reality and minds, so she could easily make Naruto lose his sanity. And since she has a grudge against the world of ninjas, making Naruto, one of the most powerful ninjas, go insane would be a great way for her to start her reign of terror all over again.
Well, one possible plot could be that Naruto is cursed by an enemy during a battle. This curse takes away his voice. As a result, he has to find new ways to communicate with his friends and allies. Maybe he starts using sign language, which he has to learn quickly. This could lead to some really interesting character development as he struggles to convey his thoughts and emotions without speaking.
One possible plot could be that Naruto is put under a powerful genjutsu. This genjutsu messes with his mind and slowly drains his emotions. As a result, he becomes detached from his friends and his sense of purpose. He no longer shows excitement for training or fighting, and his usual determination fades away.
One possible plot could be that Voldemort's lover discovers his true dark nature and decides to leave him. This shatters Voldemort as he realizes he has lost the one person who saw something good in him. It could lead to a downward spiral in his character, making him even more bitter and vengeful towards the world.
In such a fanfiction, one possible plot could be that Naruto is exposed to some dark magic or a powerful curse that gradually erodes his humanity. This might lead to him becoming cold and detached from his friends. He could start making decisions that go against his normal moral code, perhaps being more ruthless in battle or in his relationships. The yaoi aspect might come into play if another male character tries to bring back his humanity through a deep emotional and physical connection with Naruto.
Often in such fanfictions, the plot may involve a betrayal that pushes Harry over the edge. It could be a friend or a mentor who turns on him. This sudden shift in loyalty would shatter his world view. He has always relied on the support of those around him, and when that is taken away, it becomes difficult for him to stay sane. His actions would then become erratic as he struggles to come to terms with this new and cruel reality.
Maybe in the fanfic, there's a powerful villain who has a device that can nullify superpowers. Beast Boy gets caught in the range of this device and loses his powers. He struggles with feelings of uselessness at first. But then he realizes that his skills as a strategist and his knowledge of the Titans' operations are still valuable, and he uses those to help the team in a different way while also searching for a power restoration method.