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.
One way could be through a kekkei genkai transfer jutsu. If a powerful ninja, perhaps an old hermit with knowledge of ancient jutsus, decides to transfer Sasuke's Sharingan to Naruto for the greater good of the village. This jutsu might be risky but in the world of fanfiction, anything is possible.
In some fanfictions, Naruto gets the Sharingan as a gift from a dying Uchiha. This Uchiha, realizing Naruto's importance in uniting the ninja world, gives him the Sharingan. Naruto then uses it to understand the Uchiha's pain better and tries to bridge the gap between the Uchiha and the rest of the village. He also has to train hard to master the different levels of the Sharingan, like the Mangekyo Sharingan which comes with its own set of risks and rewards.
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.
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.
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.
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.
The Sharingan in a Naruto fanfic can be a really interesting element. It often adds depth to the story. Writers can explore different aspects of it, like the unique powers of different levels of Sharingan, such as the Sharingan's ability to copy jutsus. It can also be used to show the character's growth. For example, a character might start with a single - tomoe Sharingan and progress to the Mangekyo Sharingan, which could be a significant plot point in the fanfic.
One reason could be some sort of experiment. Maybe Orochimaru or another villain was trying to create a super - ninja and combined the genetic material of the Uchiha with Naruto's. Another possibility is that it's a gift from a powerful being. For example, a god - like entity could have bestowed the Sharingan on Naruto to help him in his quest to bring peace to the ninja world.