Naruto would likely become more introspective. The Mangekyo Sharingan often comes with a lot of emotional baggage in the Naruto universe. So, Naruto might start to question his own identity more, especially since this power is from a clan he has no blood relation to. He could also become more cautious as he learns about the potential dangers of misusing the Mangekyo Sharingan.
His relationships would change significantly. His friends, especially Sasuke, would view him differently. Sasuke might feel a sense of kinship in a new way since Naruto now has a part of his clan's power. Naruto would also have to deal with the envy or suspicion of some in the village who don't understand how he got the Mangekyo Sharingan. He would have to prove that he can be trusted with this powerful ability.
Naruto's fighting style would evolve. He would incorporate the abilities of the Mangekyo Sharingan into his combat techniques. He might use the illusions created by Tsukuyomi to outwit his opponents or the powerful black flames of Amaterasu to gain an upper hand in battles. This would make him an even more formidable ninja, but also one that has to constantly balance the new power with his existing values and beliefs.
It could be an original character. Maybe a young ninja from a small, forgotten clan who discovers they have the Mangekyo Sharingan. This character would have to learn about their power, its limits, and how to use it in a world full of powerful shinobi.
Another concept is the character development. The presence of the Eternal Mangekyo Sharingan often brings a deeper level of conflict and growth for the characters. They might have to struggle with the power it bestows, such as the risk of going blind or the moral implications of using its more sinister abilities.
Sasuke Uchiha is often a central character. Since he already has the Sharingan in the original series, in fanfiction with the Eternal Mangekyo Sharingan, he can be further developed. His journey to master this new level of power and how it affects his relationships, especially with Naruto, can be a major focus.
One possible plotline could be Naruto unlocking the Mangekyo Sharingan through some strange and unforeseen means, perhaps a powerful jutsu backfires and gives him this new power. Then he has to struggle with controlling it while his friends are both amazed and a bit worried about this new development in him.
His character would become more complex. With these new powers, he might face new moral dilemmas. For example, the power of the Rinnegan could tempt him to use it in ways that might not be in line with his usual values.
Naruto's character would likely become more complex. He'd have to deal with new powers and the responsibilities that come with them. He might become more introspective as he tries to understand the nature of the Sharingan and how it fits into his ninja beliefs.
Naruto's character would likely become more serious and focused. Training with Shisui and the Sharingan would demand a lot of concentration, so he'd have to tone down his wilder side. He might also become more strategic in combat, as the Sharingan gives insights into opponents' moves.
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.
His confidence might grow faster. Since the Sharingan gives an edge in combat, Naruto would likely win more battles early on, which would boost his self - esteem. He might also become more introspective as the Sharingan has some psychological aspects to it.
After awakening the Sharingan, Naruto may become more perceptive. He starts to notice small details in his surroundings and in people's behavior that he missed before. This helps him in his ninja missions as he can plan better strategies. His confidence also grows as he realizes he has a new and powerful tool at his disposal.