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.
His relationship with Sasuke would be greatly affected. Sasuke might see Naruto as more of an equal in terms of their eye powers. They could have a new level of understanding or even rivalry. It might also make Sasuke more protective of Naruto in a strange way, since the Sharingan is precious to the Uchiha.
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.
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.
In this fanfiction, Naruto would become extremely powerful. With the Sharingan, he could gain abilities like predicting movements and copying jutsus more effectively. The Rinnegan would give him powers such as the Six Paths techniques. He might be able to use these newfound powers to better protect Konoha and face stronger enemies.
In such a fanfiction, one possible plot could be that Naruto acquires the Mangekyou Sharingan through some kind of magical or experimental accident. Maybe a powerful enemy uses a jutsu that backfires and transfers the Sharingan to Naruto. Then Naruto has to learn to control its power while also facing the prejudice from those who fear the Sharingan's abilities.
Well, it might be that there's a powerful enemy who can only be defeated if Naruto has the Sharingan. Sasuke, realizing this, decides to transfer his Sharingan to Naruto. In the fanfiction, Naruto would then have to deal with the new power. He might struggle at first to control it as it's not his natural ability. But as time goes on, he could use it to better understand Sasuke's perspective and strengthen their bond even more. He could also use it in battle, perhaps seeing his opponents' moves more clearly and countering them more effectively.
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.