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.
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.
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.
In such a fanfiction, one possible plot could be that Naruto acquires the Mangekyou Sharingan through some kind of magical or experimental accident. Maybe a powerful enemy uses a jutsu that backfires and transfers the Sharingan to Naruto. Then Naruto has to learn to control its power while also facing the prejudice from those who fear the Sharingan's abilities.
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.
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.
A cool plot could be that a new villain emerges who has the ability to nullify the powers of the Sharingan and Rinnegan. Naruto and Sasuke have to find a way to overcome this without relying solely on their eye powers. They might discover ancient techniques that their predecessors used before the existence of these eye powers.
If Naruto gets the Sharingan in this fanfiction, he might unlock the power to use some of the Uchiha - specific jutsu. For example, he could potentially use the Fire Style: Great Fireball Jutsu, which is a common Uchiha technique. Also, the Sharingan has different levels of evolution like the Mangekyo Sharingan. If Naruto were to reach that stage, he could gain even more powerful abilities, such as the ability to use Susanoo, which would make him an extremely powerful force in combat.
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.
One way could be through a kekkei genkai transfer jutsu. If a powerful ninja, perhaps an old hermit with knowledge of ancient jutsus, decides to transfer Sasuke's Sharingan to Naruto for the greater good of the village. This jutsu might be risky but in the world of fanfiction, anything is possible.
Perhaps in the fanfiction, Sakura gets the Sharingan as a gift from a mysterious figure. This figure might be an ancient ninja spirit or something. With the Sharingan, Sakura's combat abilities would skyrocket. She could start seeing the chakra flow of her opponents more clearly, allowing her to predict their moves better. Also, she could use the Sharingan's genjutsu - related powers to her advantage in battles.