One interesting concept could be new and unique jutsu. For example, the author might create a jutsu that combines the powers of the eternal mangekyo in a way that has never been seen in the original Naruto series. Another concept could be different character relationships. Maybe in the fanfiction, characters who were enemies in the original become allies, all because of the power of the eternal mangekyo.
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.
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.
Naruto is usually a main focus, as the title implies it has to do with his connection to the eternal mangekyo. Sasuke is also often a key character. Their relationship and how it changes due to Naruto having the eternal mangekyo powers can be a major part of the story.
One common theme is power struggle. When a Harry Potter character gets the Eternal Mangekyo Sharingan, they have to deal with the new power and how it affects their relationships. Another theme is identity. They might question who they are now that they have this new ability from a different universe. For example, a Gryffindor might struggle with the more 'ninja - like' aspects of the Sharingan that seem at odds with the Gryffindor values.
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.
In the novels, Kakashi's evolution is a gradual process. Initially, he is a very reserved and mysterious figure. His Sharingan eye and his combat skills are what people first notice about him. But as the story progresses, his relationships, especially with Naruto, Sasuke and Sakura, start to change him. He becomes more of a guiding figure, sharing his wisdom and experiences. He also overcomes his past fears and insecurities, which is shown through his actions and decisions as the story unfolds. This evolution makes him a more well - rounded and beloved character in the Naruto universe.
At first, Kakashi might try to distance himself from Team 7's possessiveness. He values his independence and doesn't want to feel smothered. However, as he realizes that their actions are coming from a place of deep love and loyalty, he starts to accept it more. He may even start to rely on them a little bit, for example, allowing them to help him with his paperwork or other small tasks that he usually does alone. This shows a new level of trust and closeness between them.
Kakashi would likely be shown as a loving and protective husband. He would still be the skilled ninja we know, but he would also have a soft side for his OC spouse. He might share his past traumas with her and let her in on his more vulnerable side.
Another plot could revolve around a character who has the Mangekyo Sharingan but is trying to hide it from both their allies and enemies. They fear the power it holds and the attention it might draw. As the story progresses, they are forced to use it in a crucial battle, and their friends then have to deal with the revelation and the potential danger it brings to the group.