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.
Well, it could be a number of things. Maybe she faces some challenges, has a major life change, or discovers a hidden truth. But without reading the specific novel, it's hard to say for sure.
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.
In some fan theories, it could be due to a unique chakra interaction. Maybe the Kyuubi's powerful chakra somehow mutates Naruto's own ocular abilities and triggers the emergence of the Sharingan. But this is just a wild fanfiction idea.
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.