Perhaps a dark entity from another dimension. This entity might sense Yami's power and want to harness it for its own purposes, so it kidnaps him. There could be some sort of inter - dimensional struggle going on where Yami is a key piece.
One possible plotline could be that Yami is kidnapped by a rival group who wants to use his special powers for their own evil deeds. His friends then embark on a dangerous mission to rescue him. Another could be that Yami is kidnapped as part of a ransom plot against someone he is close to, and they have to figure out how to meet the kidnappers' demands without sacrificing too much.
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.
One plot idea could be Yami going on a solo mission to a mysterious and dangerous land. He has to face powerful new enemies and discover secrets about his own past. Another could be Yami training a young and inexperienced magic knight, sharing his unique combat and magic knowledge.
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 write a believable characterisation of Yami, focus on his rough exterior but also his loyalty to his squad. Show his love for food, especially Japanese food. Also, don't forget his unique magic style and how he uses it in combat.
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.
One possible fanfiction could start like this: In the dimly lit tomb, Yami Bakura's spirit stirred. He could sense a new presence in the world of the living that was somehow connected to his ancient past. A young girl, with an air of mystery around her, had stumbled upon an artifact that was once part of his dark collection. Yami Bakura decided to reach out to her in her dreams, slowly luring her into his world of shadows and secrets.
A possessive Yami can create a sense of mystery in the fanfiction. Readers might wonder why he is so possessive. Is it love? Loyalty? Or something else? This can keep them engaged and eager to find out more about his motives.