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.
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.
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.
One idea could be exploring how Itachi uses his Eternal Mangekyou Sharingan to change the past. Maybe he goes back in time to prevent the Uchiha massacre and has to deal with the consequences of altering such a major event in the ninja world.
One possible plot twist could be that Naruto gains the Mangekyou Sharingan in a moment of extreme emotional turmoil, like seeing Sasuke in a life - threatening situation. This new power might make him an outcast in Konoha at first, as the Sharingan is a symbol of the Uchiha. Another twist could be that the Mangekyou Sharingan has some unforeseen side effects on Naruto's chakra system, making it difficult for him to control his normal jutsu. It could also lead to a power struggle within the village as some might fear Naruto's new - found power.
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.
First, give the character a clear motivation related to Itachi's Eternal Mangekyou Sharingan. For example, if the character is an enemy, they could be after the power of the Sharingan. If they are an ally, they might be interested in helping Itachi control or understand the full extent of his new power.
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 this fanfiction, Naruto getting the eternal mangekyou sharingan might lead to him exploring his family's past more deeply. Since the sharingan has ties to the Uchiha clan, Naruto could discover some long - lost secrets about his own family's connection to the Uchiha. This could open up a whole new plotline where he uncovers hidden villages or techniques related to his family's history.
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.