Naruto's fighting style would evolve significantly. He would no longer just rely on his brute strength and the Nine - Tails' chakra. With the Eternal Mangekyou Sharingan, he could use powerful genjutsu and ninjutsu unique to the Sharingan. He might start to use more stealth and precision in his attacks. His training would also change as he has to master the new abilities. He would probably study Uchiha history and techniques more, which would make him more well - rounded as a ninja. This new power would also bring out a more serious side of Naruto, as he now has the responsibility of using such a powerful ability for the good of the village and the ninja world.
In Naruto fanfiction, the Mangekyou Sharingan is sometimes used to create new backstories for characters. For example, a character who didn't have the Sharingan in the original might be given one in the fanfiction, and the story could explore how they came to possess and master the Mangekyou Sharingan. This can add a whole new layer of intrigue to the character.
His personality could become extremely cold and calculating. The power of the Mangekyou Sharingan might make him see others as mere pawns. He no longer values the bonds he once had in Naruto's normal state. Instead, he is only focused on achieving his own dark goals, whether it's power, revenge, or domination.
For his relationship with Kakashi, it could be complex. Kakashi, having his own experiences with the Sharingan, would be in a unique position to guide Naruto. But at the same time, he might be worried that Naruto's new power could attract unwanted attention. Naruto's bond with the villagers could also be affected. Some might see him as more of an Uchiha than a Uzumaki now, and there could be those who fear his power. His relationship with Jiraiya, if he were still alive in the fanfic, would also change. Jiraiya would need to teach Naruto how to use this power for good and not let it corrupt him.
With Sasuke, their relationship would become more complex. Sasuke might be both jealous and intrigued by Naruto having the Mangekyou Sharingan. Jealous because it was always an Uchiha thing, and intrigued because he wonders how Naruto will use it. This could lead to more intense rivalries and also more opportunities for them to understand each other better.
In many Naruto fanfictions, Itachi is often depicted as an extremely powerful Mangekyou Sharingan user. His original abilities in the canon, like Amaterasu, are further enhanced in fan works. Sasuke is also a common focus. His Mangekyou Sharingan's powers can vary greatly from one fanfiction to another, sometimes with new and unique jutsu created by the authors. Another character could be Madara. Fanfiction writers love to explore the full potential of his Mangekyou Sharingan, often giving him abilities that were only hinted at in the original series.
In Naruto fanfiction, it's often portrayed as extremely powerful. It can give characters abilities like seeing through any illusion, predicting an opponent's moves with near - perfect accuracy.
Naruto's relationship with his friends in the Hidden Leaf Village would also change. Some might be in awe of his new power, while others might be a bit afraid. For example, Sakura might be worried that the Mangekyou Sharingan could change Naruto's personality. Naruto would have to reassure them that he's still the same person. And he might also use his new power to protect them more effectively, strengthening their bond.
One creative plot twist could be a character who was thought to be a normal ninja suddenly reveals they have a Mangekyou Sharingan with a unique power that changes the whole power dynamic in the story. For example, a side character who has been helping the main character all along, but then uses their Mangekyou Sharingan to stop an impending doom that no one else could. Another could be that the Mangekyou Sharingan has the ability to swap the powers of other ninjas, creating chaos and new alliances.
Naruto's relationship with the elders of Konoha would also be affected. The elders might see him as more of a threat now, especially if they don't fully understand the nature of his Mangekyou Sharingan. They could try to keep him in line or even restrict his movements. However, Naruto, with his new power, might be more confident in standing up to them and demanding changes in the village's policies. This could lead to a power struggle within Konoha, with Naruto at the center, trying to balance his newfound power and his loyalty to the village.