One possible fanfiction could be that Naruto is on a mission and encounters a powerful Uchiha who, instead of killing him, transfers his Sharingan to Naruto for some mysterious reason. Naruto then has to learn to control this new power while still maintaining his own unique ninja style.
In a fanfiction, Naruto might get the Sharingan through a scientific experiment gone wrong in Orochimaru's lair. He then has to deal with the sudden influx of new abilities and the strange visions that come with the Sharingan. He might also face the wrath of the Uchiha clan for having what they consider their sacred dojutsu. This could lead to an interesting plot where Naruto has to prove himself worthy of the Sharingan and use it for good.
Perhaps in a fanfiction world, Naruto gets the Sharingan when he travels back in time and meets an ancestor of the Uchiha. The ancestor, seeing Naruto's potential and pure heart, bestows the Sharingan upon him. Naruto then has to figure out how to use it in his own time period. He may face challenges from both his friends and enemies who are either jealous or suspicious of his new power.
One possible fanfiction could be where Naruto, through some strange and unforeseen circumstances, like a powerful jutsu gone wrong that somehow transfers Sasuke's Sharingan to him. Naruto then uses this new - found power in ways that make Sasuke respect him more. For example, Naruto might use the Sharingan's ability to predict movements to counter an enemy's attack and save Sasuke, leading to a stronger bond between them.
In the Naruto universe, while it's highly unlikely in the official canon, in fanfiction, anything can be made possible. The Sharingan is a kekkei genkai exclusive to the Uchiha clan. Danzo had Sharingan in his arm which he obtained through rather unethical means. For Naruto to get it from him in fanfiction, it could be written through some sort of experimental jutsu gone wrong or right. Maybe a jutsu that was meant to transfer power ended up giving Naruto the Sharingan. However, this would completely change Naruto's power dynamics and character arc as the Sharingan comes with its own set of abilities like the ability to copy jutsu, predict movements, and use genjutsu.
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.
Well, a plot might be that Naruto is experimented on by Orochimaru. Orochimaru, always interested in powerful abilities, tries to implant the Sharingan into Naruto. Naruto initially struggles with this new power as it's not native to him. But with his strong will, he begins to master it and uses it in ways no Uchiha has before. He could combine the Sharingan's visual prowess with his Rasengan, creating an even more powerful attack. For example, he might be able to see the chakra flow within an enemy more clearly with the Sharingan and then direct his Rasengan to target the weakest points.
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.
He might have increased visual abilities. For example, he could see chakra flow more clearly, which would be a great advantage in battle.
In a fanfic, Naruto getting Sasuke's Sharingan could be part of a plot where Sasuke sacrifices himself to save Naruto. Before dying, he gives Naruto his Sharingan. Naruto then has to deal with the new power as well as the emotional weight of carrying a part of Sasuke. He could use the Sharingan to see through illusions more easily and even gain some of Sasuke's memories related to the Sharingan's usage.
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.
A plot twist could be that Naruto receives the Mangekyou Sharingan from Sasuke in a self - sacrificing moment. Sasuke, realizing the true potential of Naruto's power combined with the Sharingan, decides to transfer his Mangekyou Sharingan to Naruto. This would create a whole new dynamic in their relationship. Naruto would then have to master the abilities of the Sharingan quickly to face an impending and powerful threat to the Hidden Leaf Village.
One possible plotline could be Naruto suddenly gaining the Sharingan through some mysterious jutsu accident. With the Sharingan, he could quickly learn new techniques and have enhanced perception. Combined with the Kyuubi's power, he might become an almost unbeatable force. For example, he could use the Sharingan to better control the Kyuubi's chakra flow, making his attacks more precise and powerful.