It might be Obito. He was a significant character with the Sharingan in Naruto. In the fanfiction, another character could be tricking him to gain an advantage in a fight or to prevent him from achieving his goals. For example, a character could pretend to be on his side and then betray him at a crucial moment when his Sharingan - related abilities were being counted on.
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.
Maybe by creating false visual illusions. For example, using some basic ninja techniques like the Transformation Jutsu to disguise as someone important to the Sharingan user. This could make the Sharingan user misjudge the situation and fall into the trap.
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.
In fanfiction, Naruto could get the Sharingan through a blood - related connection that is revealed later. For example, it could be discovered that he has some distant Uchiha ancestry that gets activated somehow. Maybe a powerful enemy's attack awakens this dormant Uchiha blood in him, and as a result, he develops the Sharingan. It could also be that a powerful sage or a new character uses their own unique powers to bestow the Sharingan upon Naruto for a specific purpose.
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.
If Naruto had the Sharingan in fanfiction, it would likely lead to some interesting power - up scenarios. He could combine the power of the Sharingan with his own massive chakra reserves. This might make him an even more formidable opponent. His training would also take a new turn as he'd have to learn to control and master the Sharingan's abilities, like the different levels of the Sharingan and their unique powers.
Naruto might adapt by first trying to understand the basic functions of the Sharingan. He could start with the simple ability to see chakra flow more clearly. Since he has always been perceptive about people's emotions, he could use the Sharingan to read the emotions of his enemies through their chakra, which would help him in battles.
One drawback could be the strain on his body. The Sharingan is a powerful dojutsu, and Naruto's body, which is used to his own chakra system, might not be able to handle it well at first. This could lead to fatigue and over - exertion during battles.
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.
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.
One main theme is often power. Exploring the potential power of Rin's Sharingan, how it compares to other Sharingan in the Naruto world. Another theme can be sacrifice. Given Rin's fate in the original series, fanfictions may explore the sacrifices related to her Sharingan, like she might have sacrificed her own well - being to protect others with its power.