Perhaps the Kyuubi is able to plant false memories in Naruto's mind as part of making him insane. This could lead to Naruto believing his friends are his enemies and vice versa. He might then go on a quest to 'destroy' those he thinks are threats, all while his real friends are desperately trying to reach him and bring him back to his senses.
One possible plot could be that the Kyuubi's chakra constantly overwhelms Naruto's mind, leading him to make rash decisions in battles. For example, he might attack his allies by mistake when the insanity takes over.
Well, for one thing, it allows for a fresh take on the Naruto universe. The insane aspect of Naruto can lead to different plotlines compared to the traditional Naruto stories. And the Fem Kyuubi adds an extra layer of intrigue as she has her own set of powers and personality traits. Their relationship can be explored in ways that are not possible in the original series, like how they might interact during battles or how they come to understand each other on a deeper level. It gives writers a chance to be really creative and come up with new story arcs and character developments.
A plot could involve Naruto being cursed with some kind of insanity - causing him to act erratically. Hinata, being in love with him, refuses to give up on him. She tries to find a cure while also dealing with the changed Naruto. Their relationship could become strained as Naruto struggles with his inner turmoil and Hinata tries to hold on to the Naruto she knew. But in the end, through their love and Hinata's determination, Naruto overcomes the insanity.
One interesting plot could be Naruto using his unique bond with Kyuubi to help it regain its lost memories. Naruto might explore the Kyuubi's past and discover things that even the Kyuubi itself had forgotten. This could lead to new powers or a deeper understanding between them.
In some fanfictions, the Kyuubi's protection might lead to Naruto having a special bond with nature. Since the Kyuubi is a powerful force, it could influence Naruto's chakra in a way that makes him more attuned to the natural elements around him. This could give him an edge in battles, like being able to manipulate the environment in ways others can't.
One interesting plot could be the Kyuubi's influence on Naruto's relationships in the harem. For example, the Kyuubi's power might attract some characters who are interested in that power, leading to unique interactions. Another plot could be Naruto having to balance his newfound harem with his ninja duties, and the Kyuubi sometimes interfering or helping in these situations.
Another plot could be Kyuubi teaching Naruto some ancient and forgotten ninja techniques. As a powerful and ancient being, Kyuubi might know some techniques that have been lost over time. Naruto could then use these new techniques to surprise his opponents in battles and also contribute to the growth of the ninja arts in the village.
One interesting plot could be Naruto learning to control the Kyuubi's power in unique ways that are different from the original story. For example, he might be able to use the Kyuubi's chakra to create new jutsu that are a combination of his own skills and the Kyuubi's raw power.
It shows Naruto's vulnerability. Usually, he is seen as strong - willed, but in this fanfiction, the Kyuubi's influence exposes his weakness when it comes to his mental state.
One possible plot could be Naruto gradually realizing his feelings for Fem Kyuubi as they go through various adventures together. Maybe they start off as friends but as they face more and more challenges, like fighting powerful enemies, their bond deepens into love.
In some naruto x girl kyuubi fanfiction, the plot might revolve around the girl being an outcast because of her Kyuubi - like powers. Naruto, who has always been an outcast himself, helps her accept herself. They then train together to master their powers and become a formidable duo in the ninja world. This could also lead to a romantic relationship as they understand each other on a very deep level.