One common theme is the power dynamic. Yami is a powerful character, and often in these fanfictions, the OC has to navigate around his strength and authority. For example, in many stories, the OC might be a new recruit and has to prove herself to Yami.
Asta would be a great character to pair with Yami in a crossover fanfiction. Their mentor - mentee relationship in the original series could be further explored in a new, crossover setting. Another good choice could be Naruto. Naruto's positive attitude and determination could contrast with Yami's more stoic nature, and they could learn a lot from each other in terms of combat and leadership.
One idea could be a crossover with a modern - day detective story. Yami Sukehiro could use his magic and combat skills to solve mysterious crimes in a big city. Another could be a crossover with a fantasy world where he has to adapt to new magical systems and form alliances with local heroes to fight a common evil.
Well, 'The Dark and the Light' is another one. It delves deep into the personalities of both Yami and the OC. The OC is a healer, and her interactions with the tough - minded Yami create a lot of drama and tension, which makes for a great read.
In some yami and seto fanfictions, Yami might be depicted as the more mysterious and powerful one. Seto, on the other hand, is often shown as the proud and wealthy character. Their interactions can range from intense rivalries to unexpected alliances or even romantic undertones. However, in lemon fanfictions, these characteristics can sometimes be exaggerated for the sake of the plot.
Another interesting concept could be a crossover fanfiction. Picture a situation where Luffy from One Piece, who has the Yami Yami no Mi in his world, somehow ends up in the Naruto world. This could lead to all sorts of chaos and interesting interactions as the two different power systems collide.
To create a unique Naruto Yami Yami no Mi fanfiction, start by thinking outside the box. Don't just copy the typical uses of the Yami Yami no Mi from One Piece. Instead, adapt it to fit the Naruto world. For instance, instead of just absorbing physical things like in One Piece, the power could be used to absorb ninjutsu and turn it into something else.
One of his friends could be the cause, accidentally. Maybe a friend makes a deal with a shady character without realizing the consequences. This deal ends up putting everyone in danger, and Yami gets mad at his friend's naivete and the mess they've created.
Yami remains alive in the manga. The author chose to keep Yami as an ongoing character to drive the story forward and interact with other characters in various scenarios.