One possible plot twist could be that Naruto gains the Mangekyou Sharingan in a moment of extreme emotional turmoil, like seeing Sasuke in a life - threatening situation. This new power might make him an outcast in Konoha at first, as the Sharingan is a symbol of the Uchiha. Another twist could be that the Mangekyou Sharingan has some unforeseen side effects on Naruto's chakra system, making it difficult for him to control his normal jutsu. It could also lead to a power struggle within the village as some might fear Naruto's new - found power.
As for his relationship with the rest of the village, some might be suspicious at first. The Sharingan has been associated with the Uchiha clan, which has had some dark moments. But as Naruto shows that he can use the power for good, they would gradually accept it. However, there would always be some who are wary, which could create some interesting social dynamics in the village.
Maybe in the fanfic, a dark force is trying to corrupt Sasuke's Sharingan. Sasuke, realizing he can't control it on his own, gives it to Naruto. Naruto, with his strong will, battles the dark influence within the Sharingan. He uses his own positive energy and friendship bonds to purify the Sharingan. Eventually, he masters it and uses it to bring light back to the ninja world, also using it to reunite with Sasuke in a more powerful and united way.
For his relationship with Kakashi, it could be complex. Kakashi, having his own experiences with the Sharingan, would be in a unique position to guide Naruto. But at the same time, he might be worried that Naruto's new power could attract unwanted attention. Naruto's bond with the villagers could also be affected. Some might see him as more of an Uchiha than a Uzumaki now, and there could be those who fear his power. His relationship with Jiraiya, if he were still alive in the fanfic, would also change. Jiraiya would need to teach Naruto how to use this power for good and not let it corrupt him.
I'm not sure specifically what that particular fanfic is about as there could be many interpretations. It might involve Sakura from the Naruto universe somehow obtaining the Sharingan, which is a powerful dojutsu. Maybe it could be about her newfound abilities, how it changes her relationships, or her role in battles with this new power.
In some fanfictions, it could be through a special jutsu or ritual. Maybe Itachi, before his death, transferred his Sharingan to Naruto in a unique way, perhaps using a forbidden technique that was known only to a few in the Uchiha clan.
In some fan theories, it could be due to a unique chakra interaction. Maybe the Kyuubi's powerful chakra somehow mutates Naruto's own ocular abilities and triggers the emergence of the Sharingan. But this is just a wild fanfiction idea.
In some fanfictions, it could be due to a special jutsu gone wrong. Maybe a jutsu that Kakashi was using to transfer some of his chakra to Naruto for a mission accidentally transferred his Sharingan power as well.
The Sharingan in a Naruto fanfic can be a really interesting element. It often adds depth to the story. Writers can explore different aspects of it, like the unique powers of different levels of Sharingan, such as the Sharingan's ability to copy jutsus. It can also be used to show the character's growth. For example, a character might start with a single - tomoe Sharingan and progress to the Mangekyo Sharingan, which could be a significant plot point in the fanfic.