Itachi, even though he is a very skilled Sharingan user. Naruto might come up with a clever plan to fool him. Maybe Naruto could use his understanding of Itachi's past and his relationship with Sasuke to create a situation where Itachi misinterprets what's going on. Also, other Uchiha clan members who rely heavily on the Sharingan could be victims. Naruto could study their common traits and weaknesses related to the Sharingan and use that knowledge to deceive them.
Kakashi, who has the Sharingan as well. He is used to relying on it for various things like combat and perception. Naruto could devise a strategy that takes advantage of Kakashi's trust in the Sharingan. For instance, Naruto could create a diversion that seems like a normal threat but is actually a trap set up to make Kakashi use his Sharingan in a way that leads to his downfall. And any other ninja in the Naruto universe who has the Sharingan or admires it too much could potentially be the victims of Naruto's fooling in this fanfiction.
One possibility is that Naruto uses his wits and unique skills to deceive those who overly rely on the Sharingan. Maybe he creates false scenarios or uses his own abilities in a way that misleads Sharingan users into making wrong judgments.
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.
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.
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 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.
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. He has always been interested in powerful abilities like the Sharingan. In this fanfiction, he might try to steal Naruto's newly acquired Sharingan for his own experiments and to increase his power even more.
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.
It could be a rogue ninja from a forgotten clan who has a vendetta against the Hidden Leaf. This ninja might be seeking to obtain the power of the Mangekyou Sharingan for himself, and sees Naruto as a threat. So, he tries to stop Naruto from fully awakening it and uses all kinds of dark jutsu to attack Naruto and his friends.
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.