In such a fanfic, the plot might involve Naruto growing up with the influence of Karma. Maybe he gains unique powers from it. For example, his physical abilities could be enhanced, allowing him to perform jutsus more easily. Also, his understanding of the ninja world could be different, as Karma might bring some knowledge from another realm. He could form new relationships based on this new power source, and face different challenges compared to the original Naruto story.
In this kind of fanfic, Naruto's character might develop in a way that he is more attuned to the spiritual side of the ninja world. Since Karma has some sort of otherworldly origin, he could start to see things from a different perspective. He might be more empathetic towards enemies who also have strange powers, as he understands the burden of having something that others don't understand. His growth would be about not only physical and ninja skills but also about his understanding of different forces at play in the world.
One idea could be that Karma gets transported to the Naruto world. He could start off as an unknown entity and gradually get involved in the shinobi battles. Maybe he'd team up with Naruto and use his unique skills from Assassination Classroom to help in missions.
The unique feature is the new take on karma. In normal Naruto, karma has certain implications, but in these fanfics, it's twisted. So, characters' fates and powers can be very different.
I really liked 'The Karma's Influence' fanfic. It delved deep into how Naruto's Karma affected his chakra control and his ability to perform new jutsus. The author did a great job of explaining the mechanics of Karma in the context of Naruto's powers.
A great Naruto Karma fanfic might be set in an alternate universe where Karma is much more powerful than in the original series. Naruto has to struggle not only with the power itself but also with the dark forces that are attracted to it. This could lead to him forming new alliances and having to make tough choices to protect his world.
Don't forget about the setting. If the story is mostly in the Naruto world, describe it vividly for readers who may not be as familiar with it. If it's in Karma's world, explain how Naruto adapts. Also, make sure the pacing is right. Don't rush the relationship development between Karma and Naruto, let it build up slowly and naturally over the course of the story.
The Karma Seal can change Naruto's appearance. In some fanfictions, it might give him a more menacing look with strange markings spreading across his body.
One popular 'Twisted Karma Naruto Fanfic' could be 'The Twisted Path of Naruto'. It often takes the concept of karma in Naruto and twists it in interesting ways, exploring how different characters are affected by the altered karma.