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.
One common plot is that Naruto gains the Rinnegan through some sort of ancient ritual or prophecy. Another is that it's a result of a unique chakra fusion, maybe with an Uchiha character. Often, once he has it, he has to hide it from his friends at first because they might not understand or be afraid of it.
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 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.
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.
One popular one might be those that explore how Naruto's new power of the Rinnegan changes his relationships in the harem. For example, it could show how he uses the Rinnegan's abilities to protect the various characters in his harem during battles, adding a new layer of drama and action to the story.
Kakashi can be quite popular too. His wisdom and experience could play a role in guiding Naruto as he deals with the power of the Rinnegan. Also, fans might be interested in seeing how Kakashi reacts to Naruto's new - found power and how it might affect their bond.
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.