Kakashi could also be a great choice. Since he already has the Sharingan in one eye, a fanfiction could explore more in - depth about his relationship with that single Sharingan. Maybe there are hidden powers within his Sharingan that he has yet to discover. The story could focus on Kakashi's past and how the Sharingan has influenced his life and decisions. It could also show him training a new generation of ninja while also dealing with his own Sharingan - related issues.
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 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 might also be Itachi. Imagine a scenario where Itachi's actions before his death somehow lead to his Sharingan being abandoned or lost in a different way than in the original story. This could open up a whole new narrative about his legacy and the impact it has on the Naruto world.
Sakura could be an unexpected character to have the Sharingan. It would completely change her character dynamics. She could use it to enhance her medical ninjutsu, seeing the internal body structure more clearly. Also, she could become a more formidable fighter, using Sharingan - based genjutsu to her advantage.
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.
Another option could be Kakashi. Suppose he is banished because of some incident related to his Sharingan. He has so much history and experience, and seeing him outside of the village structure, relying only on his Sharingan and his wits, would make for a great story. He could also have flashbacks to his past experiences with the Sharingan while dealing with the challenges of being banished.
A character who is a hybrid of two different clans, one with the Shikotsumyaku and the other with the Sharingan. This would give them a unique perspective and set of skills, making for an interesting main character.
It could be Sasuke. Since he already has the Sharingan and later acquires the Rinnegan, he has a natural connection to both powerful eyes. A story could focus on his further adventures and how he uses the combined powers of the eyes in different situations.
It could be Sasuke. In many Naruto - related stories, Sasuke has the Sharingan and is often the target of others' tricks or manipulations due to his complex personality and his power.
A new character could be introduced as a Sharingan master. For example, a wandering ninja who has been secretly studying the Sharingan for years. He might have no allegiance to any village and travels around, looking for worthy pupils to inherit his knowledge. He could have unique techniques related to the Sharingan that no one has seen before.
It could be Orochimaru. Since he has always been interested in obtaining powerful abilities, he might see Naruto with the Sharingan as a threat and try to take it for himself.