In the fanfiction world, it could be due to a deep emotional bond between Naruto and Sasuke. Their connection could be so strong that in a moment of extreme danger or a spiritual awakening, Sasuke's Sharingan essence is transferred to Naruto. For example, if Sasuke was on the verge of death and his chakra merged with Naruto's in a desperate attempt to save Naruto, the Sharingan could be part of that transfer. This would also add a whole new layer to their relationship and Naruto's character development as he would have to learn to control this new power.
The Sharingan might also give Naruto the ability to enter people's minds, at least to some extent. In the Naruto universe, the Sharingan has been shown to have some mind - related abilities. Naruto could use this to communicate with his friends in a more direct way, or perhaps even to break through genjutsu more easily. And, if he can enter the minds of his enemies, he could gather valuable information about their plans and weaknesses.
Well, it might be that there's a powerful enemy who can only be defeated if Naruto has the Sharingan. Sasuke, realizing this, decides to transfer his Sharingan to Naruto. In the fanfiction, Naruto would then have to deal with the new power. He might struggle at first to control it as it's not his natural ability. But as time goes on, he could use it to better understand Sasuke's perspective and strengthen their bond even more. He could also use it in battle, perhaps seeing his opponents' moves more clearly and countering them more effectively.
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.
In a fanfiction where Naruto kills Sasuke and obtains the Sharingan, it would be a game - changer for the story. Naruto would gain new abilities like enhanced visual perception and the potential to use some of the Uchiha's signature jutsu. However, this would also create a lot of internal conflict within Naruto. His ninja way has always been about saving his friends, not killing them. So, this act would make him question his own identity and values. Moreover, the political situation in the ninja world would be in chaos. The Uchiha clan's power being transferred to Naruto, an outsider in terms of the Uchiha, would be a controversial matter that could spark new rivalries and alliances.
The relationship would be deeply transformed. Naruto getting Sasuke's Sharingan means he now has a connection to Sasuke's clan and heritage. This could make Sasuke see Naruto in a new light, not just as a friend but as someone who can carry on a part of the Uchiha legacy. For Naruto, he would feel a greater responsibility towards Sasuke and the Uchiha name. They would have to work together more closely to figure out how to handle this new situation, and it could bring them closer than ever, with a new level of mutual understanding and respect.
In such a fanfiction, an interesting plot point could be that Naruto's chakra starts to mix with the Sharingan's power in unexpected ways. This could lead to the creation of a new type of Sharingan or a new power altogether. Maybe Naruto's Uzumaki bloodline affects the Sharingan in a way that no one expected. Also, the power struggle within Naruto as he tries to balance his own values with the new power of the Sharingan could be a great source of internal conflict and an interesting plot point.