In fanfiction, a great plot could be Naruto's Kyuubi form being merged with some other powerful chakra source. For example, if it combines with the natural energy of a sacred forest, it could create a new and even more powerful form. Also, Naruto could use his Kyuubi form to enter the minds of other characters and help them overcome their inner demons, as the Kyuubi itself is a symbol of a powerful inner force.
One plot idea could be Naruto and the Kyuubi fox form teaming up to face a new, powerful enemy that neither could defeat alone. This would require them to fully trust and understand each other's powers.
One idea could be that the Kyuubi in human form becomes Naruto's mentor in secret. He teaches Naruto some ancient and powerful jutsu that were lost over time. This could give Naruto an edge in battles and also deepen his connection with the Kyuubi.
One idea could be exploring the bond between Naruto and the Kyuubi in a new way. For example, they could have a shared dream where they are in a strange, alternate world and have to work together to find a way back. Another plot could involve Naruto using the Kyuubi's power to save a village from a new, powerful enemy that no one else can defeat. And a third could be about the Kyuubi revealing a long - lost secret about Naruto's family that changes his perspective on his own powers.
One plot idea could be that Naruto and Kyuubi have to work together to stop an ancient evil that is threatening the ninja world. This would require them to fully trust each other and combine their powers in new and unique ways.
One idea could be Naruto exploring the ancient powers of the Kyuubi that were long forgotten. He could discover hidden techniques that only the Kyuubi knew in its long existence.
Perhaps a plot where the female Kyuubi has to take on a human form to protect Naruto from a new, powerful enemy. In the process, they develop feelings for each other while fighting side by side.
Another plot could involve the Kyuubi having a more sentient and family - like relationship with Naruto's family. It could communicate with them in dreams and offer advice or warnings. For example, it might warn them about upcoming threats to the village that Naruto alone might not be able to handle. This would create a unique dynamic where the family has to work with the Kyuubi for the greater good.
One idea could be that the Kyuubi's power somehow interacts with Itachi's Sharingan in a unique way. Maybe Itachi is able to use the Kyuubi's chakra to enhance his genjutsu even further. Another plot could involve Naruto, with the Kyuubi inside him, teaming up with Itachi to face a common enemy that neither could defeat alone. For example, a new powerful Otsutsuki clan member arrives, and their combined skills are needed to save the village.
One idea could be Dark Naruto forming an uneasy alliance with the Kyuubi. They might decide to take on the Hidden Leaf Village together, believing it has wronged Naruto in some way. Naruto could use the Kyuubi's power in a more sinister way than ever before.
Another plot could involve Naruto sharing his Kyuubi cloak powers with his friends. He discovers a way to temporarily transfer some of the cloak's power to them, and they use it to fight a new and powerful enemy that was too strong for them before. This would also strengthen their bonds and teamwork.