It could be that Naruto is constantly put in situations where he has to make impossible choices. For instance, having to choose between saving his friends or the village multiple times, and every time he makes a choice, there are horrible consequences. Over time, the guilt and stress of these decisions build up and cause him to lose his sanity as he feels like he can't do anything right.
One possible plot could be that Naruto experiences repeated and extreme betrayals. For example, if his friends in Konoha turn on him due to false accusations of him being a threat. This would gradually chip away at his mental state, making him question his values and identity, and slowly drive him insane as he feels isolated and persecuted.
It could be an unknown force that manipulates the events around Naruto to drive him insane. This force could be an ancient evil that has a vendetta against the Uzumaki clan. It secretly plants seeds of doubt and chaos in Naruto's life, causing him to question everything he knows and trusts, slowly pushing him over the edge.
One possible plot could be that Naruto is pushed over the edge by the constant betrayal he faces. For example, if the village he has always protected turns against him due to false accusations of being a threat. He might start using his powers in a more erratic and dangerous way, not caring about the consequences. This could lead to a showdown between him and his former friends who now see him as a menace.
In some cases, it could be a new and powerful enemy. This enemy might have the ability to manipulate chakra in a way that affects Naruto's mind. For example, they could send out waves of negative chakra that slowly erode Naruto's mental state. Over time, these attacks make Naruto go insane as he struggles to fight against this unseen and insidious force.
The Akatsuki as a whole could be the opponents. Imagine Naruto facing multiple Akatsuki members at once in a fight. The combined threat of their different powers and their goals of capturing the tailed beasts could be overwhelming for Naruto. They might use underhanded tactics, like threatening to kill captives or using mind - control techniques on Naruto's friends. All of these things could make Naruto go insane as he battles against them, trying to protect what he loves.
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 in this fanfiction, Naruto is exposed to a powerful dark jutsu that slowly corrupts his mind. As he becomes insane, he begins to abandon the values he once held dear. He could start using his powers for self - gain rather than for the protection of the village, like taking over control of Konoha in a tyrannical way.
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.
He might end up hurting his friends. In the Naruto series, Naruto values his bonds deeply. But if he's losing control, his actions could become unpredictable and cause harm to those close to him.
I'm not entirely sure specifically what a 'naruto league insane fanfic' is about without reading it. But generally, it could be a fan - created fictional story within the Naruto universe that might have some really wild or extreme ideas, perhaps involving unique battles, unexpected character relationships, or crazy plot twists.
Well, perhaps the shrinking is a result of some magic gone wrong. As she shrinks, she loses her confidence at first. But then, she realizes that her new small size gives her an advantage to explore hidden corners of Wonderland. She could find secret passages or solve mysteries that were impossible for her at her normal size.