Well, in a fanfiction where Naruto is trained by Itachi in Sharingan, Naruto would first have to adapt to the new power. Itachi would probably start with the basics of how to activate and control the Sharingan. Since Naruto is used to his own physical and chakra - based training, this would be a challenge. However, with Itachi's guidance, he could master it. Once he does, he could use the Sharingan to analyze his opponents' fighting styles better. For example, he could quickly figure out the weaknesses in an enemy's jutsu and counter them effectively. He could also use the Sharingan's visual prowess to enhance his reaction time during battles.
Naruto's character development in this fanfiction scenario would be multi - faceted. Firstly, his relationship with Itachi would be a major factor. He'd respect Itachi more and perhaps see him as a mentor. As he trains in the Sharingan, he'd learn discipline in a new way, different from his training with Jiraiya. His interactions with other characters, especially those who are wary of the Sharingan's power, would change. He might have to prove that he can use this power for good. His growth would not only be in terms of combat skills but also in his maturity and how he views the ninja world as a whole.
One aspect of the plot could be Naruto's struggle to control the new power. The Godlike Sharingan is a powerful and potentially dangerous ability. Naruto might initially have a hard time adjusting to seeing the world through the Sharingan's perspective. Itachi would then have to guide him through this process. Also, they could face enemies who are after the power of the Godlike Sharingan that Naruto now has, leading to intense battles where Naruto has to use his new skills and the knowledge Itachi has imparted to him.
In some fanfictions, it could be through a special jutsu or ritual. Maybe Itachi, before his death, transferred his Sharingan to Naruto in a unique way, perhaps using a forbidden technique that was known only to a few in the Uchiha clan.
One possible fanfiction could be that Itachi, on his deathbed, decides to transfer his Sharingan to Naruto. He sees in Naruto the potential to change the Uchiha's fate and the ninja world. Naruto, with the Sharingan, gains new abilities like enhanced perception and the ability to copy some jutsus more easily. He uses this new power to better understand the Uchiha and bridge the gap between them and the rest of the village.
Naruto's character would develop a new sense of self - confidence. Having the Godlike Sharingan and being trained by Itachi would give him a feeling of being a part of something greater. He would no longer be just the 'loud - mouthed underdog' but a powerful ninja with unique abilities. However, he would also have to deal with the new expectations that come with this power. He might struggle with the idea of whether he is still the same Naruto or if the power has changed him too much. This internal conflict would be an important part of his character development as he tries to find a balance between his old self and the new, more powerful version of himself.
They could develop a mentor - mentee relationship. Itachi uses his knowledge of the Sharingan to teach Naruto new combat techniques. As they train together, they bond over their shared experiences of loss and their desire to protect the people they care about. Naruto might also help Itachi come to terms with his past actions and find some peace within himself.
One possible plot could be that Itachi, seeing Naruto's potential and his importance for the future of the Hidden Leaf, decides to secretly train him. Itachi might teach Naruto advanced jutsu, especially those related to chakra control and unique Sharingan - inspired techniques. Naruto, under Itachi's tutelage, would grow not only in power but also in his understanding of the ninja world's complex politics and the Uchiha clan's history.
Itachi is such an interesting character in the Naruto series. His Sharingan abilities are cool and powerful. In fanfiction, writers can expand on his story, his relationships, and his inner thoughts. For example, they can explore his relationship with Sasuke in new ways or show how he developed his Sharingan skills. Also, the idea of Itachi as a tragic hero makes people want to read more about him in fanfiction.