Character development is key. The female Kyuubi should have her own distinct personality. She could be sassy, wise, or mischievous. Naruto should also stay true to his character, being determined and having a strong sense of justice. The plot should be engaging, with some sort of conflict, like an external threat to the village or a problem within the relationship between Naruto and the Kyuubi. Good fanfiction also often has great descriptions of the Naruto world, whether it's the landscapes or the unique jutsu. And it should add something new to the existing Naruto story, not just be a rehash of what we already know.
A good fanfiction in this category should have a well - developed relationship between Naruto and the female Kyuubi. It should show their growth and how they influence each other. For example, Naruto could learn more about his own power from the Kyuubi, and the Kyuubi could become more empathetic towards humans through Naruto. Also, the story should be set in the Naruto universe with accurate lore. It can't just randomly change the rules of chakra or how tailed beasts work.