Itachi could have realized Naruto's potential and determination. He knows Naruto is different from other ninjas and has a unique ability to change people's hearts. By giving Naruto his Sharingan, Itachi might be ensuring that Naruto has the power to carry out some important mission or to protect the people Itachi cares about.
One possible reason could be that Kakashi is in a life - threatening situation and realizes that Naruto is the best candidate to inherit his Sharingan. He knows Naruto's potential and his desire to protect the village. Another reason could be that Kakashi wants to see how Naruto would use the Sharingan in combination with his own unique abilities. He's curious about the new powers and techniques that Naruto could create with it. Also, it could be a way for Kakashi to show his complete trust in Naruto, as the Sharingan is a very precious and powerful gift.
One reason could be some sort of experiment. Maybe Orochimaru or another villain was trying to create a super - ninja and combined the genetic material of the Uchiha with Naruto's. Another possibility is that it's a gift from a powerful being. For example, a god - like entity could have bestowed the Sharingan on Naruto to help him in his quest to bring peace to the ninja world.
One reason could be Naruto's power. In the Naruto universe, Naruto has great power, and both Itachi and Sasuke might see Naruto as a key to achieving certain goals. Itachi might want to protect Naruto because he sees Naruto as a force for good in the future of the ninja world. Sasuke, on the other hand, may view Naruto as a rival in terms of power and influence, and in fanfiction, this could lead to a fight between them over Naruto.
In some fanfictions, it could be due to a powerful emotional shock. Maybe seeing a friend in great danger at Wave, like Sasuke almost dying, could trigger an unexpected connection to the Sharingan. Since Naruto has a strong bond with Sasuke, this extreme situation might force his latent Uchiha - like powers (in the context of the fanfic) to surface, awakening the Sharingan.
Well, the plot could start with Itachi realizing that Naruto is the key to the future of Konoha and the ninja world. So, he gives Naruto his Sharingan. Naruto then has to deal with the new power. He might start having visions related to Itachi's past, which could help him understand the Uchiha - Senju conflict better. And this new knowledge could influence his decisions in future battles and political situations in the ninja world.
One possible reason could be for Naruto's protection. His parents might think that by giving away his birthright, they can keep him away from dangerous enemies or political intrigue in the ninja world. Maybe they foresee a great threat that would target Naruto specifically because of his birthright, and this is their extreme measure to ensure his survival.
A simple plot could be that Itachi entrusts Naruto with his Sharingan because he believes Naruto can use it to bring peace to the ninja world. Naruto, being the kind - hearted person he is, accepts the responsibility. He starts training with Kakashi to understand and master the Sharingan's powers. With the Sharingan, Naruto can anticipate his opponents' moves better and uses this advantage in battles against powerful enemies like Pain.
One reason could be for strategic purposes. Itachi was a highly strategic ninja. So, in fanfiction, Naruto might act like him to outwit a particularly difficult enemy. For instance, when facing an enemy who is a master of deception, Naruto could adopt Itachi's calm and calculating demeanor to see through the enemy's tricks.
One possible scenario in such a fanfic could be that Itachi, seeing Naruto's potential and his pure heart, decides to entrust Naruto with his Sharingan. This could potentially give Naruto new abilities and a deeper understanding of the Uchiha clan's power. It might also create a unique bond between Naruto and the memory of Itachi, leading to new storylines where Naruto uses the Sharingan's powers in unexpected ways.
One possible consequence is that Naruto would gain enhanced visual abilities. The Sharingan can see chakra flow clearly, which would be a great asset in battle for Naruto. He could better analyze his opponents' moves and weaknesses.