In many Naruto fanfictions, Itachi is often depicted as an extremely powerful Mangekyou Sharingan user. His original abilities in the canon, like Amaterasu, are further enhanced in fan works. Sasuke is also a common focus. His Mangekyou Sharingan's powers can vary greatly from one fanfiction to another, sometimes with new and unique jutsu created by the authors. Another character could be Madara. Fanfiction writers love to explore the full potential of his Mangekyou Sharingan, often giving him abilities that were only hinted at in the original series.
Another common type of main character is a reincarnation of an ancient Sharingan user. In one fanfiction, Rei is a reincarnation of a long - dead Sharingan master. Her Sharingan has the power to see the future, and she uses this to try and prevent a great war from breaking out in the Naruto world. She teams up with Naruto and Sasuke in this story.
One great 'Naruto Most Powerful Sharingan Fanfiction' is 'The Ascent of the Sharingan God'. In this story, the main character with the most powerful Sharingan goes on a journey to master all its abilities and defeat the ultimate evil. It's full of intense battles and character development.
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.
It would probably be Sasuke. His Sharingan gives him so many abilities like enhanced perception, the ability to copy moves, and eventually the Mangekyo Sharingan powers. In the My Hero Academia world, these skills would be extremely powerful. He could easily outwit and overpower most of the heroes and villains there.
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.