Perhaps in a fanfiction, Fuu is a key to unlocking the true potential of Naruto's Rinnegan. She has a special chakra that when combined with Naruto's Rinnegan energy, creates a new and powerful jutsu. This jutsu could be the only thing that can stop an impending doom that threatens the ninja world.
One idea could be that Fuu discovers a hidden power within Naruto's Rinnegan that can open a portal to a different dimension filled with ancient ninja secrets. She then teams up with Naruto to explore this new world and face the challenges within.
A fanfiction could revolve around a young ninja who is born with both the Sharingan and the Rinnegan. He has to hide his powers from the world as he is hunted by those who want to use his powers for evil. As he grows, he meets Naruto and they form an unlikely alliance. Together, they train to master his unique abilities and fight against the forces that seek to destroy them.
One idea could be a story where Naruto somehow acquires the Rinnegan and combines its powers with the Susanoo. Maybe he has to face an extremely powerful enemy that can only be defeated by this new, super - powered combination. He could start by accidentally awakening the Rinnegan during a battle and then discovers that he can merge it with his Susanoo - like form.
One idea could be a story where a character discovers a way to combine the powers of the Rinnegan and Sharingan. For example, they might be able to use the Sharingan's precognitive abilities to enhance the Rinnegan's control over the six paths. This could lead to new and unique jutsu that no one has ever seen before.
One idea could be a story where Naruto discovers a hidden power within his Rinnegan that allows him to communicate with the Shinigami. Maybe he needs to make a deal with the Shinigami to save his friends from a powerful enemy. The Shinigami could offer Naruto a special jutsu in exchange for something precious to him.
A fanfiction could center around a battle between two powerful ninjas, one who has mastered the Mokuton and Sharingan, and the other who has the Rinnegan. The fight could be not just about physical combat but also about the clash of different ideologies related to these powers. The one with the Mokuton and Sharingan might believe in a more traditional view of power, while the Rinnegan user could have a more radical view.
One idea could be a story where Naruto somehow acquires the Mangekyou Sharingan and Rinnegan. He then has to learn to control these new powers while facing an ancient evil that only he can defeat with the combined might of these dojutsu. Maybe he teams up with Sasuke, who also has his own power upgrades, and they have to figure out how to merge their abilities to save the ninja world.
One idea could be a story where Sasuke, with his Sharingan, discovers a new power related to the Rinnegan that was hidden within Naruto all along. Maybe Naruto has some dormant Sage of Six Paths energy that when combined with Sasuke's powers, they can unlock a new level of strength to fight a powerful, unknown enemy.
Naruto's Rinnegan might allow him to sense Fuu's presence from afar, and Fuu's agility could be enhanced by the Rinnegan's power. For example, if they are in a battle, Naruto could use his Rinnegan to mark the enemies, and Fuu could quickly dart between them, taking them out one by one.
One idea could be that Azula somehow gets transported to the Naruto universe and is given the power of the Rinnegan. She might use it in her own cunning way, perhaps to gain more power over the Hidden Villages. She could manipulate the new powers with her already devious nature, creating a new and dangerous force in the Naruto world.
One popular type of Naruto fanfiction involving the Rinnegan is where Naruto gets the Rinnegan early on in the story. This changes the entire course of the plot as he becomes super - powerful from the start. Another popular one is where Naruto combines the power of the Nine - Tails with his Rinnegan abilities in unique ways, like using the Nine - Tails' chakra to enhance the Rinnegan's long - range attacks. There are also fanfictions that focus on Naruto using the Rinnegan to travel between different dimensions, which is really exciting as it introduces new worlds and adventures.