It could also mean that Naruto's actions, which seem normal on the surface, are actually part of a more complex and insane plan in the context of the fanfiction. For example, his pursuit of becoming Hokage might have a much darker and more insane motivation that he keeps secret.
It could be that Naruto discovers some dark secrets about his past, such as the real reason his parents died or some hidden and malicious intentions of the village elders. This overwhelming knowledge could be too much for him to bear and push him over the edge into insanity. He would then start to act erratically, causing turmoil in the ninja world.
Perhaps in the fanfiction, some sort of dark magic or curse has been placed on Naruto. This could be messing with his mind and causing him to act in an insane manner. It could be a way for the author to introduce a new and dark plot element to the story. Another cause might be that he has been betrayed by those he trusts the most, which shatters his mental state.
A plot could involve Naruto being secretly depressed because he realizes the cycle of hatred in the ninja world may never end. He's fought so hard to bring peace, but new threats keep emerging. He starts to question the purpose of his efforts and falls into a deep depression. He may keep this hidden from others because he doesn't want to discourage them or seem weak, but it slowly eats away at him.
To develop this concept further, the fanfiction could show how Naruto's insanity affects his relationships. His friends might start to distance themselves when they sense something wrong, or some might try to help him, which could lead to a lot of emotional and action - filled scenes.
Well, one scenario could be that Kushina finds a fanfiction where Naruto has turned evil due to some dark force. Another might be that she discovers a story where Naruto is in a relationship that is completely out of character for him. Maybe she stumbles upon a fanfiction where Naruto has some really strange and overpowered abilities that don't fit the original Naruto universe.
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.
One possible plot development could be that Naruto discovers his deafness gradually and has to hide it from his enemies. He might be afraid that they would use it against him in battle. Another could be that his friends find out by accident and have to learn sign language to communicate better with him.
One storyline could be Naruto gradually losing his sight over time but hiding it because he doesn't want to worry his friends or seem weak. He could struggle with simple tasks like reading scrolls or finding his way around Konoha without making it obvious.
Well, there could be a 'naruto insane naruto fanfiction' where Naruto teams up with his enemies in a very unexpected way. Maybe he forms an alliance with Orochimaru not for power, but to save the entire ninja world from a new, greater threat. They would have to put aside their differences and work together, which would be a really insane concept considering their past relationship.
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.