One common element could be a distorted view of the characters' personalities. For example, Naruto might be portrayed as more erratic or self - destructive. Another might be a darker, more hopeless world - building compared to the original Naruto universe.
I'm not really into promoting fanfics that involve mentally unstable themes as they can often be quite dark and disturbing. However, you can try searching on some major fanfic platforms like FanFiction.net or Archive of Our Own and use the relevant tags to look for such works, but be cautious as they might not be suitable for all readers.
One possible reason could be the extreme pressure he faces in the fanfic. Maybe he has to deal with much more powerful enemies alone, which overwhelms his mental state. Also, if he experiences a lot of betrayals in the fanfic, like friends turning against him, it could lead to his mental instability.
One example could be a fanfic where Harry experiences extreme mood swings. Maybe he suddenly goes from being overly excited to deeply depressed within a short period, and his friends in the story are constantly worried about his erratic behavior.
Bind the Nation's Fate: Me! Mental illness! Teammate Sister Bao 'er "is for you. The story revolved around a strange game. The protagonist, Wu Tian, had transmigrated to a parallel world and was bound by the system to have a mental illness. He and a little loli became teammates on the national team. The countries mocked them for not having any hope for them, but when they entered the game and faced various situations, they showed their superior strength. I hope you like my recommendation. Muah ~
In some fanfictions, Naruto might be depicted as mentally unstable for various reasons. One could be the extreme stress he endures. He has lost many people in his life, like his parents at a young age. This could potentially lead to psychological trauma that some fanfic authors might explore and exaggerate, making him seem mentally unstable.
An effective 'harry is mentally unstable fanfic' can be written by developing strong supporting characters. These characters can react realistically to Harry's condition. For instance, some friends might be understanding and try to help, while others might be afraid or judgmental. This adds depth to the story. Also, the author should use Harry's inner thoughts to show his mental state. If he's constantly doubting himself or feeling paranoid, these thoughts can be written in a stream - of - consciousness style to let the readers really understand his instability.
It adds more depth to the genre. By showing Naruto as mentally unstable, it allows for different types of stories. For example, stories about his recovery or his struggle with his mental state can be created, which makes the fanfiction more diverse.
The fem Kyuubi could be initially attracted to Naruto's struggle with the Kyuubi inside him. She decides to help him. As Naruto's mental state improves due to her efforts, they could develop a romantic relationship. For example, she could teach him how to better control the Kyuubi's chakra, and in the process, they fall in love.
Maybe Naruto's mental instability is a result of suppressing the Kyuubi for so long. The fem Kyuubi fanfic could explore how she tries to break through his mental barriers. She could use her own unique connection to the Kyuubi's power to reach Naruto on a subconscious level. This could lead to a series of adventures where Naruto has to face his inner demons with her help.