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.
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.
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.
One interesting concept could be how Kakashi uses the eternal Mangekyou Sharingan's unique powers in new and unexpected ways. For example, he might be able to create more complex illusions or have enhanced teleportation abilities.
The fanfiction might show Kakashi growing in terms of his understanding of the Sharingan's true nature. As he delves deeper into the abilities of the eternal Mangekyou Sharingan, he could gain a broader perspective on the Uchiha and their powers, which in turn enriches his own character development. He may start to see things from an Uchiha - like point of view, which was previously foreign to him. Also, the new power might attract new enemies or allies, and through these interactions, Kakashi will have to adapt and grow. For example, if new enemies target him because of his powerful Sharingan, he'll need to find ways to outwit them and protect those he cares about. This will surely contribute to his character growth.
Kakashi's character would likely become more confident in his abilities. With the Dual Mangekyou Sharingan, he has access to greater power, which could boost his self - assurance. He might take on more dangerous missions without hesitation.
Perhaps a storyline where Kakashi's dual Mangekyou Sharingan makes him a target. Enemies from other villages might see his new power as a threat and try to either steal his eyes or kill him. This could lead to a series of intense battles and chase scenes. Kakashi would then have to rely on his wits and the help of his friends and comrades to stay alive. In the process, he could form new alliances and strengthen old ones, all while protecting his newfound 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.
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.
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.