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 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.
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.
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 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 possible fanfiction could be that Itachi, on his deathbed, decides to transfer his Sharingan to Naruto. He sees in Naruto the potential to change the Uchiha's fate and the ninja world. Naruto, with the Sharingan, gains new abilities like enhanced perception and the ability to copy some jutsus more easily. He uses this new power to better understand the Uchiha and bridge the gap between them and the rest of the village.
Naruto's perspective on the ninja world could change. With the Rinnegan, he would see things in a different light, both literally and metaphorically. He might have to deal with new enemies who are attracted to his new power or new responsibilities that come with such a powerful dojutsu. His relationships with his friends and allies could also be affected as they may view him differently now that he has this extremely powerful ability.
They could develop a mentor - mentee relationship. Itachi uses his knowledge of the Sharingan to teach Naruto new combat techniques. As they train together, they bond over their shared experiences of loss and their desire to protect the people they care about. Naruto might also help Itachi come to terms with his past actions and find some peace within himself.
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.