One way could be through a powerful jutsu gone wrong. Maybe a jutsu that was meant to transfer Itachi's memories to Naruto accidentally transferred his Sharingan as well. In the world of fanfiction, there are often many creative and out - of - the - box explanations.
In some fanfictions, it could be through a special jutsu or ritual. Maybe Itachi, before his death, transferred his Sharingan to Naruto in a unique way, perhaps using a forbidden technique that was known only to a few in the Uchiha clan.
One way this could occur in the fanfiction is if there was some sort of genetic manipulation. Maybe a rogue ninja or a mad scientist found a way to combine Uchiha genes with Naruto's. Mikoto, being an Uchiha, had the Sharingan, and through this unethical genetic process, Naruto ended up with the Sharingan. This would also open up a whole new set of abilities and challenges for Naruto as he has to learn to control this new power which is very different from his normal chakra - based powers.
In the fanfiction world, it could be due to a deep emotional bond between Naruto and Sasuke. Their connection could be so strong that in a moment of extreme danger or a spiritual awakening, Sasuke's Sharingan essence is transferred to Naruto. For example, if Sasuke was on the verge of death and his chakra merged with Naruto's in a desperate attempt to save Naruto, the Sharingan could be part of that transfer. This would also add a whole new layer to their relationship and Naruto's character development as he would have to learn to control this new power.
He might have increased visual abilities. For example, he could see chakra flow more clearly, which would be a great advantage in battle.
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.
In this fanfiction, Naruto might gain some unique jutsu skills from Itachi. Itachi is extremely powerful, so he could teach Naruto advanced genjutsu or ways to better control his chakra. For example, Naruto might learn how to quickly break free from strong genjutsu attacks, which would be a great advantage in battles against tough opponents.
Well, he could gain some amazing new abilities. The Sharingan is known for its copying abilities, so Naruto might be able to copy jutsus more easily. He could also potentially gain some of the Uchiha clan's unique powers related to genjutsu. This would change his combat style significantly, making him even more unpredictable to his enemies.
Well, if Naruto gets Shisui's Sharingan in a fanfiction, he could gain some really cool abilities. The Sharingan has powers like enhanced perception and the ability to copy jutsu. Naruto already has his own powerful chakra and skills, so with the Sharingan, he might become an even more formidable ninja. He could potentially use the Sharingan's genjutsu capabilities to his advantage in battles, maybe even use it to break through tough enemy defenses more easily.
In fanfiction, if Naruto got the Sharingan from Madara, it would completely change his power dynamic. He would gain the abilities of the Sharingan like enhanced perception, the ability to copy jutsu more easily, and perhaps even access to some of Madara's unique Sharingan - based techniques. This would make him an even more formidable opponent and could potentially change the entire plot of the Naruto universe within the fanfiction world.
Naruto would gain the ability to copy jutsu. With the Sharingan, he could watch an opponent's movements closely and replicate their techniques. This would be a huge advantage for him in battles.