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.
In some fanfictions, there could be a plot where Naruto's possession by the Kyuubi is a result of a dark curse or an experiment gone wrong. This could lead to a whole new adventure where he has to not only deal with the Kyuubi's power but also find a way to reverse the cause of the possession and save himself and those around him.
Another plot could involve Naruto being possessed by the Kyuubi in a more extreme way, where his personality changes and he has to struggle to keep his true self intact while dealing with the Kyuubi's influence. His friends might not recognize him at first, and he has to find a way to break free on his own.
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 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.
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 is when the Kyuubi can communicate with Naruto's friends in a 'Fox Naruto Kyuubi' fanfiction. It creates a whole new dynamic as the friends have to deal with the powerful and often grumpy Kyuubi. Another plot is the Kyuubi being able to take a physical form outside of Naruto and having its own adventures while still being linked to Naruto. It's really cool to see the Kyuubi interact with the ninja world on its own terms.
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.
Obviously, Naruto is the main character as the story revolves around him being possessed by the Kyuubi. Sakura and Sasuke could also play important roles. Sakura might be worried about Naruto and try to find a way to help him break free from the possession. Sasuke could be either an ally or an antagonist depending on the plot. He might see Naruto's new - found power as a threat or a means to achieve his own goals.
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.
A great plot could be that Fem Naruto wearing the Kyuubi attracts the attention of other villages. They see her as a potential threat or an opportunity. This could lead to diplomatic tensions. Meanwhile, within her own village, she has to deal with the elders who are worried about her newfound power and how it might disrupt the balance. But Fem Naruto, with the Kyuubi's power, is determined to prove that she can use it for the greater good. She might start training with new mentors who understand the Kyuubi better, and through her training, she unlocks even more hidden abilities.
In some fanfictions, a possible plot is that Fem Naruto uses her femininity to her advantage when dealing with the Kyuubi. She could charm or trick the Kyuubi in some way that Naruto in the original couldn't. Like, she might offer the Kyuubi something it really wants in exchange for its power, but in a more cunning way.
In some fanfictions of this type, an interesting plot could be the transformation of the harem due to Naruto's growth and new powers related to the Kyuubi. As Naruto becomes stronger, the dynamics within the harem change. New characters might be attracted, and old relationships might be re - evaluated. This could create a complex web of emotions and storylines that keep readers engaged.