In the Naruto universe, while it's highly unlikely in the official canon, in fanfiction, anything can be made possible. The Sharingan is a kekkei genkai exclusive to the Uchiha clan. Danzo had Sharingan in his arm which he obtained through rather unethical means. For Naruto to get it from him in fanfiction, it could be written through some sort of experimental jutsu gone wrong or right. Maybe a jutsu that was meant to transfer power ended up giving Naruto the Sharingan. However, this would completely change Naruto's power dynamics and character arc as the Sharingan comes with its own set of abilities like the ability to copy jutsu, predict movements, and use genjutsu.
There would be several implications. Firstly, Naruto's training would take a new turn. He would need to learn how to control and use the Sharingan properly, which could be a difficult task considering his already existing training regime. Secondly, politically, it could cause a stir in the Hidden Leaf. The Sharingan is a symbol of the Uchiha clan, and Naruto getting it from Danzo (who had a controversial relationship with the Uchiha) could lead to various political debates. Thirdly, on a personal level, Naruto might experience new emotions and temptations that come with the Sharingan's power. He might have to struggle with the darker aspects of the Sharingan, such as the potential to be consumed by its power, just as some Uchiha did in the past.
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 a fanfic where Danzo gives Naruto the Sharingan, it could lead to a very different Naruto. Naruto might gain new abilities and a new perspective on the Uchiha. He could potentially master the Sharingan quickly with his strong willpower, but he would also have to deal with the dark side of the Sharingan, like the curse of hatred that often comes with it.
Naruto would become much more powerful. The Sharingan could give him an edge in battles, allowing him to copy jutsu more easily. He might be able to use it to analyze his opponents faster. However, he would also have to deal with the negative aspects of the Sharingan, such as the potential strain on his eyes and the risk of overusing it.
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.
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.