The relationship might develop through a mutual understanding of loneliness. Both characters have had their share of isolation in different ways. Fem Naruto was an outcast as a child, and Kaguya has been alone for a long time. They could bond over this shared experience, and as they open up to each other, their relationship could become one of deep trust and companionship.
At first, it might be one of suspicion. Naruto would be wary of Kaguya's power and her past as a sort of villain. But as they face challenges together, they could start to respect each other. Naruto might see the good in Kaguya that was buried under her past actions, and Kaguya could come to admire Naruto's tenacity and his strong sense of justice.
Their relationship could develop realistically if Naruto starts by seeing the pain in Kaguya's past. He has a history of empathizing with his enemies, like Nagato. So, he could begin to understand why Kaguya did what she did. This understanding could be the first step in them forming a connection.
Their relationship might start with Kaguya being instantly drawn to Naruto because of his strength and positive energy. But as her yandere side kicks in, she becomes over - possessive. Naruto would be confused at first, but gradually try to understand her.
It could start with curiosity. Fem Naruto might be curious about the power and mystery of Futa Kyuubi. As she gets closer, she realizes that Futa Kyuubi has its own personality and desires. They then start to communicate more, which builds the foundation of their relationship.
At first, the relationship might be full of anger and resentment. Fem Sasuke could feel that Naruto has let her down. But as the story progresses, perhaps Naruto shows his true self and his reasons for his actions. This could make fem Sasuke see him in a new light. They might start to communicate more, share their dreams and fears, and gradually fall in love as they understand each other better.
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.
Their relationship could start with Naruto being curious about the fem robot. He would approach her and try to understand how she functions. As he spends more time with her, he'd start to see her as more than just a machine. The fem robot, on the other hand, would be initially cold and logical but Naruto's kindness would start to make her develop emotions, leading to a deepening bond.
One interesting plot could be that Fem Naruto, with her determination, teams up with Futa Kaguya. Maybe they are on a quest to bring peace to a land torn by war. Fem Naruto's combat skills combined with Kaguya's unique powers would make for an exciting adventure.
The relationship could start with fear. The reader, knowing of Kaguya's power, would be afraid. But as they interact more, they might find a sense of pity for Kaguya, who has been alone for so long. This could lead to a more understanding relationship.
At first, the relationship might be based on mutual benefit. The fem Kyuubi might see Naruto's mercenary skills as useful for her own goals, and Naruto might see her power as an asset. As time goes on, they could develop a deeper emotional bond. They might start to understand each other's pasts and traumas, which would bring them closer together.