One idea could be an OC with the Rinnegan who is from a hidden clan. They use their Rinnegan powers to uncover ancient secrets that could change the ninja world. For example, they might find a lost jutsu that was sealed away long ago.
Maybe an OC with the Rinnegan gets thrown into a different dimension. There, they have to use their unique eye powers to survive and find a way back home. They could encounter strange creatures and new forms of ninjutsu that they have to adapt to.
One idea could be an OC with the Rinnegan who has the power to manipulate game glitches in the virtual world of the fanfiction. They could use it to their advantage, like teleporting within the game or accessing hidden areas.
Perhaps the OC is a healer who is the only one who can keep Naruto's body stable when he overuses his Rinnegan. Their relationship starts as a necessity but then develops into a deep friendship or more. They get involved in a conflict where Naruto has to use his Rinnegan to save the OC's clan from destruction, and in the process, they learn more about the secrets of the Rinnegan and their own destinies.
One idea could be an OC with the Rinnegan being a transfer student to the Highschool DxD world. Their Rinnegan abilities could attract the attention of different factions. For example, they might be able to use the Preta Path to absorb the demonic energy attacks of the devils, which would both intrigue and alarm the devils in the school.
Well, a cool plot for a Rinnegan Naruto crossover fanfiction could be that there is a rift between the Naruto world and a world where the Rinnegan is more common. Naruto gets pulled into this other world and has to adapt to the different power system there. He meets characters who have mastered the Rinnegan and has to learn from them to find his way back home. Along the way, he discovers a connection between the two worlds that was hidden for centuries.
One plot idea could be a story where Naruto discovers he has the Mokuton and Rinnegan abilities due to some ancient curse or prophecy. He then has to learn to control these new powers while facing off against a new, powerful enemy that is immune to normal jutsu. Another idea is a fanfic where Naruto uses his Mokuton and Rinnegan to travel to different dimensions to find a way to save the Hidden Leaf from a looming disaster. Or perhaps a story where he has to team up with old enemies who are also intrigued by his new powers and together they form an uneasy alliance to fight a greater evil.
One interesting plot could be a character with the red rinnegan having the power to manipulate time on a small scale. For example, they can rewind a few seconds during a battle to avoid an attack or foresee an enemy's move. Another idea is that the red rinnegan gives the ability to communicate with ancient spirits of the Naruto world, which could lead to uncovering long - lost secrets about the origin of chakra.
One idea could be a crossover with the Harry Potter universe. Sasuke with his Rinnegan could be a mysterious new student at Hogwarts. Maybe he's on a mission to find a powerful magical artifact that has a connection to his ninja world. Another plot could involve a crossover with the DC universe. He could team up with Batman, both being brooding and powerful in their own ways. Sasuke's Rinnegan abilities would be a great addition to their crime - fighting efforts.
One plot idea could be that Naruto and Sasuke discover a new power related to the Rinnegan that can only be accessed when they combine their chakra. Another could be a time - travel plot where they go back in time to prevent the creation of the Rinnegan in the wrong hands. Maybe they have to face an enemy who has a corrupted version of the Rinnegan powers and they must use their own understanding of the power to defeat them.
One idea could be Sasuke traveling to a different dimension where the Rinnegan has different powers and he has to adapt. Maybe he meets an alternate version of himself with a unique Rinnegan ability.
Perhaps a plot where Naruto's Rinnegan is a result of a curse placed on him by an ancient evil. As Hokage, he has to not only fight against external threats but also the inner struggle of controlling the power of the Rinnegan without succumbing to the curse. He could learn about the origin of the Rinnegan from old scrolls and seek the help of sage-like characters to overcome the curse.