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.
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.
One possibility is that Naruto would gain incredible new abilities. Shisui's Sharingan had unique powers like Kotoamatsukami. With it, Naruto might be able to influence others in a more subtle way. He could use it in battles to deceive opponents or even resolve conflicts without direct combat. For example, he could use it to make enemies see illusions of peace and lay down their weapons.
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 such a fanfiction, an interesting plot point could be that Naruto's chakra starts to mix with the Sharingan's power in unexpected ways. This could lead to the creation of a new type of Sharingan or a new power altogether. Maybe Naruto's Uzumaki bloodline affects the Sharingan in a way that no one expected. Also, the power struggle within Naruto as he tries to balance his own values with the new power of the Sharingan could be a great source of internal conflict and an interesting plot point.
In a fanfiction, if Naruto gets Shisui's Sharingan, he might face consequences in his training. He would need to learn how to control and master this new power, which could take time away from his normal training. This could potentially slow down his progress in other areas, like his Rasengan training or his control over the Nine - Tails' chakra. Also, his relationships with his non - Uchiha friends might be affected. They could feel left out or less important to Naruto now that he has this powerful Uchiha ability.
One challenge is mastering the Sharingan's abilities. Since Naruto is not an Uchiha, he doesn't have the natural affinity for it. He would have to learn from scratch how to activate it, control it, and use its powers like the Sharingan - enhanced perception and the ability to copy jutsu. Another challenge could be dealing with the potential side - effects. The Sharingan can be a strain on the user's eyes and chakra. Naruto already has the Nine - Tails' chakra which can be unstable at times. Combining the two could be a tricky balance.
He could perform powerful genjutsu with the Sharingan. Naruto, who was not really known for genjutsu before, would now be able to create illusions that are difficult to break. With the Rinnegan, he might be able to use the Six Paths techniques, like the ability to control gravity or absorb chakra attacks.