Well, in such a fanfiction, a possible main plot development could be the fem demon kyuubi using her powers to control Naruto in some way. Maybe she wants to use him for her own goals, like getting access to some hidden power within him. It could also involve Naruto trying to break free from her control and regain his own agency.
Obviously, the fem demon kyuubi and Naruto are the key characters. The fem demon kyuubi is the one with the power and control in this scenario. Naruto is the one being 'owned', so his struggle against this situation is central to the story.
Another plot development could be the impact on the other characters in the Naruto universe. The presence of a fem Kyuubi might change how characters like Sasuke view Naruto. They could be more intrigued or even more hostile depending on how they perceive the new relationship between Naruto and the fem Kyuubi. Also, the village elders might react differently, perhaps with more fear or more acceptance, which would then influence the overall political and social situation in the village.
It could mean control. Maybe the fem Kyuubi has the ability to influence Naruto's actions or decisions in the fanfiction.
One possible plot development could be Naruto learning to master Samehada's unique abilities. Since Samehada can absorb chakra, Naruto might figure out a way to use this to his advantage in battles, like redirecting the absorbed chakra against his enemies.
In this fanfiction, Naruto could develop his skills in a new way. Since he is dealing with a banished Fem Kyuubi, he may have to learn different ways to communicate and work with her. This could lead to him becoming more adaptable. The Fem Kyuubi might develop from a vengeful being into one who is more forgiving. She realizes that not all humans are bad through Naruto's kindness, and this changes her view of the world and herself.
One possible plot development could be that Naruto is initially shocked by Fem Kyuubi's proposal. He might be hesitant because he has his own goals and relationships in the village. However, as the story progresses, he could start to see the good in her and consider the idea. Maybe she shows him a side of her that is kind and caring, different from the fearsome Kyuubi he knew before.
One possible plot could be that Naruto, as the demon lord, discovers his connection to the fem Kyuubi and begins to gather other characters into his harem. They could face challenges from other powerful forces in the Naruto world who oppose the idea of a demon lord and his harem. Along the way, Naruto has to balance his newfound power and his relationships.
A possible plot development might involve Naruto exploring his own past to understand how he became the Kyuubi's father. He could discover secrets about his own family or ancient jutsu that led to this situation. Along the way, he might face opposition from those who don't want this relationship to be known or who fear the implications it could have on the balance of power in the ninja world. This would lead to many conflicts and challenges for Naruto to overcome in the fanfiction.
Maybe Naruto gains new insights into the nature of chakra. The Kyuubi's chakra is very different from his own, and through the merge, he could understand how to manipulate chakra in ways he hadn't considered. This could lead to him teaching others new ways of using chakra as well.
One common theme is power. Since Naruto is a demon and Kyuubi is a powerful being as well, the stories often revolve around how they use and control their powers. Another theme is partnership, showing how they work together.