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.
In the fanfiction, Naruto might have an encounter with an old Uchiha who imparts some unique chakra or knowledge to him. This interaction could stir something within Naruto that has been dormant. As he struggles with the implications of this new power and the responsibility that comes with it, his eyes could awaken the Mangekyou Sharingan. For example, the Uchiha could tell Naruto about a secret power within the Sharingan that only someone with his willpower and connection to his friends could access.
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.
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.
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 possible plot twist could be that Naruto's new Mangekyou Sharingan gives him the ability to access a new form of Sage Mode. Since the Sharingan has unique powers and Sage Mode is Naruto's own power, their combination might create something unexpected. For example, he could use the Sharingan to better target his Sage chakra, making his attacks more precise and powerful.
One possible plot twist could be that Naruto's Sharingan has unique abilities not seen in the regular Uchiha Sharingan. Maybe it can combine with his Sage Mode in unexpected ways, like seeing chakra flows more clearly even in Sage Mode, enhancing his combat skills.
In this fanfiction, perhaps the awakening of the Sharingan gives Naruto an edge in battles that makes the female ninjas admire him even more. The plot might involve Naruto being initially confused by all the attention from the girls, but then gradually coming to terms with it. For example, Sakura could see him in a new light and Hinata might be even more smitten. And this new dynamic could lead to various adventures and challenges as Naruto tries to balance his relationships with his duties as a ninja.
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.
A plot twist could be that Naruto receives the Mangekyou Sharingan from Sasuke in a self - sacrificing moment. Sasuke, realizing the true potential of Naruto's power combined with the Sharingan, decides to transfer his Mangekyou Sharingan to Naruto. This would create a whole new dynamic in their relationship. Naruto would then have to master the abilities of the Sharingan quickly to face an impending and powerful threat to the Hidden Leaf Village.
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 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.