Perhaps Naruto leaves Tsunami for the Kyuubi because he senses a new threat that is related to the Kyuubi's power. He realizes that by staying with Tsunami, he might put her in danger. So, he decides to distance himself and go directly to the source of his power. He may plan to confront the Kyuubi's darker aspects and try to transform it into a more positive force. In the fanfiction, this could lead to him unlocking new abilities and forms that are based on his relationship with the Kyuubi.
In the fanfiction, Naruto might leave Tsunami for the Kyuubi due to a prophecy. The prophecy could state that only by fully embracing the Kyuubi can he save the ninja world from an upcoming catastrophe. Naruto, being the selfless hero he is, doesn't want to risk Tsunami's safety and decides to follow this path alone. He may go to places filled with ancient chakra related to the Kyuubi, like an old battlefield where the Kyuubi once rampaged. There, he starts his exploration of the Kyuubi's true nature.
One possible plot could be that Naruto leaves Tsunami in order to better understand and control the power of the Kyuubi within him. He might embark on a journey to isolated places where he can train without endangering Tsunami. For example, he could head to a secluded mountain range. There, he could focus on communicating with the Kyuubi spirit, trying to make a truce or gain more cooperation from it. This would be a significant step in his growth as a ninja and his development of the unique power he possesses.
Obviously, Naruto himself is a main character. He will be interacting with the Kyuubi. Since the Kyuubi is a powerful and sentient entity within Naruto, their interactions could be intense. Naruto might also encounter other ninja who have knowledge about the Kyuubi. For example, Jiraiya. Jiraiya, being Naruto's mentor, might appear to give advice on how to handle the situation with the Kyuubi. He could share some of his own research and experiences related to the tailed beasts.
A possible fanfiction could be set in an alternate universe where Naruto, Kyuubi, and Tsunami are all part of a prophecy. They have to journey together to find a sacred relic that can either save or destroy the ninja world. Along the way, Naruto has to constantly balance the power of the Kyuubi within him, and Tsunami uses her survival skills and knowledge of the world to help them overcome various obstacles.
In fanfiction, Tsunami might have a special ability that can soothe the Kyuubi's wild nature. Naruto, being the host of the Kyuubi, is amazed by this. They start to work together, with Tsunami using her ability regularly on the Kyuubi. This leads to a more harmonious relationship between Naruto and the Kyuubi, and also a deep friendship between Tsunami and both Naruto and the Kyuubi as they rely on each other more and more.
One possible plot could be that Naruto, instead of being shunned in the village, is secretly taken in by the Kyuubi. The Kyuubi, seeing Naruto's loneliness, decides to raise him. Naruto would grow up learning unique skills from the Kyuubi, like advanced chakra control and perhaps some secret fox - related jutsu. This would make him a very different kind of ninja compared to the original Naruto in the series.
Well, one possibility could be a very creative and out - of - the - box plot where Naruto's unique bond with the Kyuubi leads to some sort of magical or spiritual connection that results in an unexpected pregnancy. But this is a very strange and far - fetched concept in the Naruto universe as it goes against the normal biological and story elements.
The plot of a 'naruto frees kyuubi fanfiction' might involve an external threat to the village. This threat is so great that Naruto realizes he needs the full power of the Kyuubi, not just the limited access he has through the seal. So, he starts to work on breaking the seal. Along the way, he has to face opposition from some in the village who fear the unleashed Kyuubi. But Naruto is determined. He finally frees the Kyuubi and together they defeat the external threat. After that, the Kyuubi decides to stay with Naruto as a friend and ally, seeing the true nature of the young ninja.
Well, perhaps in this fanfiction, the idea is that Naruto and the Kyuubi have a spiritual mating of sorts. It might mean that Naruto gains an even deeper understanding of the Kyuubi's nature. This could involve Naruto delving into the Kyuubi's memories and learning ancient secrets that help him not only in combat but also in his journey to bring peace to the ninja world. As a result, they become an even more formidable force combined.
In such a fanfiction, Naruto might leave his village in anger and sadness. He could go on a journey of self - discovery, trying to come to terms with the betrayal. Maybe he'd meet new allies who help him grow stronger and more independent.
One possible plot could be that Kushina's departure makes Naruto more determined to prove himself. He might strive harder in his ninja training, using the pain of his mother leaving as motivation. This could lead to him achieving new levels of power at a faster rate.
In this fanfiction, Naruto might leave the underworld because he has a mission to fulfill in the real world. Maybe he has to stop a new threat that endangers both the living and the underworld.