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.
In a fanfiction where Naruto gets the Rinnegan and Sharingan, his power would skyrocket. The Sharingan's ability to predict movements and see chakra flow would be a great addition to his skill set. The Rinnegan, on the other hand, would open up a world of new possibilities. He could use the Asura Path to create mechanical weapons or the Animal Path to summon powerful creatures. This would not only make him stronger in battle but also change his role in the ninja world. He might be seen as a god - like figure, and his influence would extend far beyond just being a Hokage. He could potentially bring about a new era of peace, using his overwhelming power to deter any potential threats.
In such a fanfiction, Naruto would become an extremely powerful ninja. With the Sharingan, he could copy jutsus more effectively and gain the precognitive abilities it offers. The Byakugan would enhance his perception, allowing him to see the chakra network clearly. This could lead to new combat strategies and make him a force to be reckoned with in battles against both Akatsuki and other threats. He might also develop unique jutsus that combine the powers of both the Sharingan and Byakugan, like a new form of genjutsu that uses the Byakugan's long - range vision and the Sharingan's ability to manipulate it.