In the fanfiction, the Rinnegan Naruto could be transported to a different dimension with his sister. There, they have to figure out how to get back while also uncovering secrets about their family and the origin of the Rinnegan. This could involve them meeting strange characters and having to use their wits and powers to survive.
Maybe Naruto's sister has a special ability that is enhanced by the Rinnegan. They get involved in a conflict against an ancient evil, and their combined powers are the only hope. The fanfiction could explore how they learn to work together, with Naruto protecting his sister while she unlocks new powers.
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 a story where Naruto discovers he has a long - lost sister who has unique powers. They then team up to face a new and powerful enemy in the ninja world. Another could be a plot where the sister is from a different village and they have to overcome the differences between their villages to be together as family.
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 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.
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.
Another plot could be that Naruto's sister is in a harem - like situation where multiple guys are interested in her, but Naruto accidentally gets involved and ends up being the one the girls in the group are more attracted to. He might initially be oblivious to their feelings, thinking they are just being friendly because of his sister, but as time goes on, he realizes the true nature of their affections.
Another plot could involve a forbidden jutsu that combines the powers of these three ocular abilities. Maybe a hidden scroll is discovered that details how to perform this jutsu, but it comes with a great cost. The main character has to decide whether to pursue this power despite the risks, which could lead to internal conflict as well as conflict with other characters who oppose this dangerous path.
One interesting plot idea could be the discovery of a long - lost member of the Rinnegan Clan who has unique powers. This new character could bring a new dynamic to the Naruto universe, either as an ally or a potential threat. Another idea is exploring the origin of the Rinnegan within the clan, perhaps with a story that delves deep into the past and uncovers secrets that were long forgotten.