Power. The eternal mangekyou sharingan represents a great power in the Naruto world, so many fanfics focus on how characters use or struggle with this power.
One possible plot could be that Naruto gains the Eternal Mangekyou Sharingan through some sort of unexpected and mysterious event. Maybe it's a result of a powerful jutsu gone wrong that fuses his own chakra system with that of a Sharingan - user. This new power could initially be difficult for Naruto to control, leading to him having to isolate himself to figure it out. As he masters it, he could use it in unique ways, like combining the Sharingan's abilities with his own Rasengan techniques, creating entirely new and powerful attacks.
The plot could involve some sort of time - travel or alternate dimension scenario. Naruto could be sent to a parallel world where the Uchiha clan was never massacred. There, he befriends the Uchiha and learns their ways. Eventually, he earns the right to receive the Eternal Mangekyou Sharingan through his deeds and his strong bond with the clan. With this new power, he has to find a way to return to his own world and use it to protect his friends and village from a new, greater threat.
One popular 'naruto eternal mangekyou sharingan fanfic' is 'The Power of the Eternal Mangekyou'. It delves deep into the abilities and the backstories related to the eternal mangekyou sharingan in the Naruto universe. The characters are well - developed, and it shows how the power of this special dojutsu can change the course of battles and relationships.
A possible plot could be that Dark Naruto's Eternal Mangekyou Sharingan has a special power that can merge the minds of people. He uses this to merge with the good Naruto inside him. Through this merge, he battles his inner demons and slowly transforms back to his normal self while also gaining the combined strength of both his dark and light sides.
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.
One possible story could be that a new character enters the Chunin Exams with a Mangekyou Sharingan. This could create a huge stir among the other participants. Maybe they use their unique abilities to quickly outwit their opponents in the early rounds. For example, they could use the Mangekyou Sharingan's genjutsu powers to make their enemies see illusions of powerful attacks, causing them to make mistakes in their defenses.
His relationships would also change. Some of his friends might be a bit wary of him now that he has such a powerful and somewhat feared ability. Naruto would have to work hard to show them that he's still the same Naruto. At the same time, he could form new alliances with those who are interested in his new power or who can help him understand it better. He might also be more isolated at times as he delves deeper into mastering the Sharingan, which would be a big change from his usually outgoing and social self.
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 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.