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 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.
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 interesting fanfiction could be about a character from Naruto who somehow gets transported to the Fairy Tail universe and has to deal with the magic there while still having their EMS powers. For example, Sasuke could end up in Magnolia and form an unlikely alliance with Natsu to fight a common enemy.
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.
Well, I don't have a specific 'Harry Potter Eternal Mangekyo Sharingan' fanfiction to recommend right off the bat. But when you're looking, consider the reviews and ratings of the stories. A highly rated story is more likely to be well - written. In these fanfictions, you might expect to see things like a Harry Potter character training to master the Eternal Mangekyo Sharingan, perhaps with the help of a character who has knowledge of both the wizarding world and the Naruto universe. This could lead to some really interesting character development and plot twists as they navigate two very different power systems.
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.
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 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.