In some fanfictions, it could be due to a special jutsu gone wrong. Maybe a jutsu that Kakashi was using to transfer some of his chakra to Naruto for a mission accidentally transferred his Sharingan power as well.
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.
Well, in fanfiction, one way could be that some Kumo ninjas are given the Sharingan as a gift or a curse from an ancient being. They then start to form a special group within the village dedicated to understanding and mastering the Sharingan. This group would share their knowledge with others, and over time, the Sharingan becomes an important part of Kumo's ninja heritage, with its own set of training methods and traditions.
There could be a deeper interaction where the OC's Sharingan actually evolves due to exposure to Fairy Tail magic. Maybe the strong magical energy in the Fairy Tail world causes the Sharingan to gain new abilities, like being able to see the future more clearly in magical battles or being able to sense the presence of hidden magical items. This would create an exciting development in the fanfiction as the OC discovers and masters these new powers.
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 obtain the Golden Sharingan through a powerful ninja ritual. Maybe a sage or an ancient ninja figure passes on the power to him as a last resort to save the ninja world from a great evil. For example, if there's a threat that only the combined powers of the Uzumaki and Uchiha clans can defeat.
In Naruto fanfiction, the Eternal Mangekyou Sharingan (EMS) often enhances the Sharingan in several ways. For one, it gives more stable and continuous power. The regular Mangekyou Sharingan can cause blindness over time with excessive use, but the EMS doesn't have that problem. So, characters can use their Sharingan abilities like enhanced perception, predicting enemy moves, and powerful genjutsu more freely.
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.
You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own. Use the search bar and type in keywords like 'Naruto' and 'Sharingan'. Then, look at the ratings and reviews of the stories to find the ones that are highly regarded. Also, join Naruto - related forums or groups on social media and ask for recommendations from other fans.