There are quite a few out there. Some focus on the comical aspects of a Ranma 1/2 and Yoki crossover. For example, the fanfic where Ranma accidentally gets involved in Yoki's family drama and his attempts to get out of it using his martial arts skills and shape - changing abilities. Others might explore a more serious plot, like when they team up to face a common enemy that threatens both their worlds.
One common trope is the 'fish - out - of - water' scenario. For example, when Ranma ends up in Yoki's world and has to adapt to new rules and cultures. Another is the 'team - up - against - a - common - enemy' trope. It's always exciting to see how their different abilities come together to defeat a powerful foe. And of course, the 'romantic - subplot' where there might be some sort of love interest developing between characters from the two different franchises.
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.
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.
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.