In some fanfictions, it could be due to a special jutsu gone wrong. Maybe a jutsu that Kakashi was using to transfer some of his chakra to Naruto for a mission accidentally transferred his Sharingan power as well.
One great fanfiction could be about Kakashi using his Sharingan to train Naruto in a special jutsu. Maybe he shows Naruto how to better perceive an opponent's moves by using the Sharingan's ability to analyze. This could lead to Naruto developing a new and powerful technique that combines his own chakra control with the insights from the Sharingan.
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.
One interesting fanfic could be where Kakashi uses his Sharingan in a unique way to train Naruto. Maybe he shows Naruto how to better analyze his opponents' moves just by using the Sharingan's ability to copy and predict. For example, Kakashi could create a special training ground where Naruto has to face multiple clones of himself that mimic the fighting styles of different enemies. And the Sharingan plays a crucial role in Naruto understanding the patterns of these clones.
The consequences could be far - reaching in the fanfiction. Naruto's relationship with Kakashi might change. There could be a deeper sense of respect and a new kind of bond formed because of this transfer. In terms of power, Naruto could unlock new jutsus or abilities that are related to the Sharingan. He might be able to combine his existing Rasengan techniques with the Sharingan - enhanced perception. This could make him an even more formidable opponent. However, it could also put a strain on his body. The Sharingan is known to consume a lot of chakra, and Naruto's body might need to adapt to this new power drain. If he overuses it, it could lead to exhaustion or even long - term damage.
One great 'Naruto Kakashi's Sharingan' fanfic could be 'The Legacy of the Sharingan'. It delves deep into how Kakashi's Sharingan not only affects his combat abilities but also his relationships with other characters like Naruto. It might show how he passes on some of his knowledge related to the Sharingan to Naruto.
They're popular because Kakashi is a very popular character. His Sharingan gives him a unique and cool look. Fanfics about it allow fans to explore more about his powers, which were not fully shown in the original series. For example, fans can create scenarios where Kakashi uses his Sharingan in ways we haven't seen before.
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.
One popular theme is the exploration of Kakashi's past through his Sharingan. Writers often use the Sharingan as a tool to delve into the traumas and important events that made Kakashi who he is. For instance, they might show how the loss of his comrades is reflected in the power and limitations of his Sharingan.
One idea is that an enemy targets Kakashi's Sharingan and Naruto has to protect it. Along the way, Naruto learns more about the Sharingan's capabilities and how to use it against the enemy. This would be a great opportunity for Naruto to grow as a ninja and for their friendship to deepen.