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.
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.
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.
His relationship with Sasuke would become more complex. Sasuke, as an Uchiha, might feel threatened or intrigued by Naruto having the Sharingan. They could have a new level of rivalry or a new form of cooperation.
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.
His relationship with Sasuke would become really complex. Sasuke, as an Uchiha, might feel a mix of jealousy and concern. Jealousy because Naruto now has a part of what makes the Uchiha special, and concern because he knows the dangers of the Sharingan. They might have more intense battles or training sessions as Sasuke tries to understand how Naruto got the Sharingan and how it's affecting him.
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.
One possible triggering event could be the loss of a dear friend. Just like Sasuke's awakening of the Mangekyou Sharingan was related to the trauma of Itachi's actions. In a fanfiction, Naruto might witness the death of someone like Kakashi, who has been a mentor figure. This intense emotional pain and the sense of loss could be the catalyst for his Mangekyou Sharingan to awaken. It would add a new layer of depth to his character, as he would have to deal with this new power and the grief that came with its awakening.
Perhaps in the fanfiction, Naruto discovers a hidden connection between his Uzumaki heritage and the Sharingan. This newfound knowledge, combined with a great battle against a powerful villain, causes the Mangekyou Sharingan to awaken. It could also involve some sort of ancient prophecy or power that was long forgotten, and Naruto is the key to unlocking it which leads to the awakening of his Mangekyou Sharingan.
It could be a rogue ninja from a forgotten clan who has a vendetta against the Hidden Leaf. This ninja might be seeking to obtain the power of the Mangekyou Sharingan for himself, and sees Naruto as a threat. So, he tries to stop Naruto from fully awakening it and uses all kinds of dark jutsu to attack Naruto and his friends.
Well, normal Naruto mainly relies on his chakra and the Nine - Tails' power. But Dark Naruto with Eternal Mangekyou Sharingan would have unique visual jutsu. He could use the Sharingan's genjutsu abilities on a much greater scale, perhaps even trapping entire villages in an illusion without them realizing.