In some fanfictions, the Sharingan is used to explore the history of the Uchiha clan from a different perspective. For instance, a story could focus on a young Uchiha who is born with a defect in their Sharingan. Through their journey, they discover secrets about the origin of the Sharingan that were long forgotten by the clan, and this knowledge could potentially change the future of the Uchiha and the entire ninja world.
He might gain enhanced perception abilities, which would be a huge boost to his combat skills. With the Sharingan, he could predict his opponents' moves more accurately.
First, do your research on the Naruto universe, especially about the Sharingan. Know its powers, limitations, and the significance in the Uchiha clan. Then, create an interesting character. It could be someone new who gets the Sharingan or an existing character with a new take on their relationship with the Sharingan. For example, if you choose Naruto to have a Sharingan - like ability, how would that change his character and his relationships with others like Sasuke?
A rogue Uchiha who has lost the use of one of his Sharingan eyes due to a failed experiment. He becomes jealous of the protagonist who has one working Sharingan eye and tries to take it for himself. He might use his remaining Sharingan powers in cruel and unexpected ways to try and defeat the protagonist.
If Naruto had a Sharingan in fanfiction, it would drastically change the story. He could quickly copy enemy jutsus like Sasuke. His combat ability would skyrocket as he could predict movements better. Also, his relationships might change. For example, his bond with Sasuke could be different as they'd both have the Sharingan. It would also make his training with Kakashi more interesting as Kakashi could teach him unique Sharingan - related techniques.
A new character could be created as the main character. This character could be an orphan who discovers they have a single Sharingan eye, and they don't know where it came from. They then embark on a journey to find out about their heritage and the origin of their eye. Along the way, they meet various Naruto characters and learn about the power and responsibility that comes with the Sharingan. They might face challenges from both those who want to steal their eye and those who fear its power.
A new ability could be the use of genjutsu, which Naruto has not been proficient in before. However, the challenge would be that using the Sharingan drains his chakra more than usual. As Naruto is used to his own chakra - intensive techniques, this could put a strain on his energy reserves during long battles.
One possibility could be through a forbidden jutsu experiment gone wrong. Maybe some rogue ninja was trying to create a super - powered being and Naruto ended up being the subject. The Sharingan was transplanted into him, but only one eye was successful.
In some fanfictions, he might obtain it through a powerful jutsu gone wrong that somehow fuses Uchiha DNA with his own. Maybe during a battle where he's exposed to some strange chakra energy related to the Sharingan.
One plot idea could be a character who is an experiment and has only one Sharingan eye implanted. They struggle to control its power and are hunted by those who want to use their unique situation. For example, they might have to constantly fight off rogue ninjas who think they can steal the eye's power.