In a fanfiction, Naruto could inherit the Rinne Sharingan from a long - lost relative. This would completely change his abilities and fighting style. He could start to use the powers of the Rinnegan, like the Six Paths techniques, in combination with his own Uzumaki and Jinchuriki powers. This would make him an even more formidable opponent, and he could use these new powers to protect the Hidden Leaf Village from threats that were previously unbeatable.
One plot idea could be that Naruto discovers an ancient scroll that reveals a way to obtain the Rinne Sharingan. This new power then causes a rift in the ninja world as some fear his newfound strength while others see it as a way to finally achieve peace. He has to struggle with mastering this power while also dealing with the political implications.
Maybe Naruto gets the Rinne Sharingan during a battle with an extremely powerful and unknown enemy. The power suddenly awakens within him, and it gives him the ability to see the enemy's moves before they happen more clearly. With this, he can turn the tide of the battle. However, he soon realizes that the Rinne Sharingan has a dark side that he needs to control to avoid being consumed by it.
One possible plot could be that Naruto discovers an ancient scroll or meets a mysterious sage who reveals the secrets of obtaining the Rinne Sharingan. This new power could initially be difficult for Naruto to control, causing some chaos in his training. But as he masters it, he could use it to gain new insights into the nature of chakra and the world around him, and perhaps even find a new way to bring peace to the ninja world.
In a fanfiction with Naruto getting the Rinne - Sharingan, it could be part of a time - travel plot. Naruto is sent back in time and in this alternate timeline, he somehow gains the Rinne - Sharingan. With this new power, he tries to change the future for the better. He might use the unique abilities of the Rinne - Sharingan to prevent some of the tragedies that occurred in his original timeline, like the death of some of his comrades. But changing the past also has its own consequences and he has to deal with the butterfly effect of his actions.
Another plot could involve a crossover between Naruto's world and a character from the Uchiha clan who has the Rinne Sharingan. They could team up to face a common enemy that is threatening both their worlds. Along the way, they would learn from each other's abilities and cultures. For example, Naruto could teach the Uchiha character about the power of friendship and teamwork, while the Uchiha could show Naruto some advanced Sharingan techniques.
Another plot could revolve around a character who accidentally acquires the Rinne Sharingan and has to learn to control it while being hunted by various groups. Some might want to use the power for their own evil purposes, while others fear the unknown capabilities. This character could start as a relatively weak ninja but gradually become stronger as they master the Rinne Sharingan, forming alliances with unexpected characters along the way.
One plot idea could be a character from Naruto ending up in the Highschool DxD world and having the Rinne Sharingan. They could use its powers in the battles against the devils and fallen angels in the new world.
One plot idea could be that Naruto is on a mission and stumbles upon an ancient Uchiha artifact that somehow bestows the Sharingan upon him. He then has to quickly learn to control it while his friends are both amazed and a bit wary. Another could be that during a battle with an unknown enemy who has the power to transfer abilities, Naruto ends up with the Sharingan. This leads to him exploring the new abilities in unique ways, like combining Rasengan with Sharingan - enhanced perception.
Perhaps Naruto gets the Sharingan in a different dimension where he has to adapt to this new power. He could face challenges like being hunted by those who want to steal the Sharingan from him. He might also discover new abilities within the Sharingan that are unique to him because he's not a pure - blooded Uchiha. For example, he could use the Sharingan to enhance his Sage Mode, seeing things more clearly and predicting attacks better.
In some fanfictions, a character from another dimension enters the Naruto world with the Rinne Sharingan. This new character's power throws the balance of the Naruto world off. The story could revolve around Naruto and his friends trying to understand this new power and either make an ally of the character or defeat them to restore peace in their world.
One interesting concept could be the exploration of new jutsu combinations using the Rinne Sharingan. For example, creating unique space - time ninjutsu that surpasses what we've seen in the original series. Another might be delving into the backstory of how the Rinne Sharingan came to be in a particular fanfiction's universe, perhaps it was the result of a long - forgotten experiment.
A possible fanfiction could be about the origin of the Rinne Sharingan having some connection to the world of RWBY. Maybe there was an ancient civilization in RWBY's past that had a similar power, and through some sort of dimensional rift, it influenced the development of the Rinne Sharingan in Naruto's world. This would involve a lot of world - building and exploration of both universes' histories.