Perhaps a plot where the Rinnegan avatar is actually a reincarnation of an ancient being from both the Naruto and Avatar universes. This being has a mission to restore balance between the two worlds that have somehow become intertwined. The character would then face challenges from both sides, with some Naruto - like villains trying to stop the restoration of balance and some Avatar - style antagonists as well.
One idea could be a story where the character with the Rinnegan avatar has to navigate a world where the rules of chakra and bending are combined. For example, they might use the Rinnegan's powers to control different elements in a way similar to how avatars control them in the Avatar world.
Another interesting plot could be a story set in a future where the Naruto and Avatar worlds have merged. The main character with the Rinnegan avatar is part of a group that is trying to protect the new hybrid world from a new threat that emerged from the combination of the most dangerous elements of both universes, like powerful jutsus combined with the dark side of bending.
Well, a plot could revolve around a Rinnegan - induced time - loop situation. A character, perhaps Sasuke, gets caught in a time - loop where he has to keep reliving a certain period in the Naruto story but with different outcomes each time due to the influence of the Rinnegan. He has to figure out how to break the loop and in the process, discovers new secrets about the Rinnegan and its origin. This could also lead to him changing the fate of some characters who died in the original story.
One plot idea could be a character with red hair and the Rinnegan being a long - lost descendant of an ancient and powerful clan. They could be on a journey to discover their true heritage and in the process, face off against powerful villains who want to use their unique powers for evil.
One idea could be Naruto having an encounter with a death - like entity that grants him new and dark powers. He might struggle with the morality of using these powers while still trying to protect his village.
One plot idea could be Naruto discovers he has the Rinnegan during a particularly intense battle against a new, powerful enemy. His friends are initially shocked but quickly realize they need to adapt to this new power. Another idea is that the Rinnegan was secretly given to Naruto by an ancient entity as a last - ditch effort to save the ninja world from an impending doom that only Naruto can face with this power.
One interesting plot idea could be a story where a character with both Sharingan and Rinnegan has to face an ancient evil that threatens the entire ninja world. This character could struggle with the power and the different abilities of both eyes as they try to master them in order to defeat the enemy.
One idea could be a crossover where Naruto characters learn bending techniques from Avatar characters. For example, Naruto might try to master water bending, which could be a really cool concept as he already has control over his chakra, and adapting it to bending could create some unique fight scenes.
Maybe the Avatar, in his journey to master all the elements, travels to the Naruto world. He could learn some ninja techniques and incorporate them into his fighting style. The Avatar could also teach the ninja about the power of bending, which could lead to some really unique team - up strategies among the characters.
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.