The story might be about the OC having a special power. Kakashi is initially intrigued by this power and then falls for the person behind it. Once married, they work together to use the OC's power for the good of the village. They might also have some domestic scenes, like cooking together and having small arguments over silly things that show their normal married life.
Well, it could be that an enemy uses some sort of mind - control jutsu to force them into marriage as part of a cruel plan. In the fanfiction, Sakura might use her medical ninjutsu skills to try and find a way to break the forced bond while Kakashi uses his Sharingan - like skills to analyze the situation. Their friends in the village would be in shock, and some might try to help them while others might be suspicious of this sudden union. As they are forced to live together, they discover new sides of each other that they hadn't noticed before.
One possible plot could be that they got married in a spur - of - the - moment decision during a dangerous mission. They keep it a secret because they fear the reactions of their friends and the village. As the story progresses, they struggle to balance their secret relationship with their duties in the village.
One possible plot could be that after years of fighting side by side and growing closer, Kakashi finally realizes his deep feelings for Sakura. They start dating secretly at first, not wanting to disrupt the village's harmony. Then, as their relationship blossoms, they decide to get married. Their wedding could be a grand event in the village, with all their friends and comrades coming to celebrate. It could also show how they face challenges together as a married couple, like dealing with new threats to the village while also maintaining their relationship.
Kakashi would likely be shown as a loving and protective husband. He would still be the skilled ninja we know, but he would also have a soft side for his OC spouse. He might share his past traumas with her and let her in on his more vulnerable side.
One possible plot twist could be that Anko is initially hesitant about the marriage due to her past traumas. Maybe she has some secrets she's afraid to share with Kakashi. For example, her past association with Orochimaru might make her doubt her own worthiness in a relationship. Another plot twist could be external threats to their marriage. The shinobi world is full of danger, and perhaps an enemy targets Anko to get to Kakashi, or vice versa. This could put a strain on their relationship as they try to protect each other while also dealing with the normal stresses of a new marriage.
One possible plot could be that Rin and Kakashi are married and they play a role in Naruto's growth. Maybe they pass on some of their skills and wisdom to Naruto. For example, Kakashi could train Naruto in new jutsus, and Rin could help with medical ninjutsu training.
In such fanfiction, the Kakashi OC often has unique physical features. For example, they might have distinct hair color or eye shape that sets them apart.
One possible plot twist could be that their relationship is initially opposed by the village elders. They might see it as unconventional or worry about how it could affect Naruto's role as a potential Hokage. Another twist could be that an old enemy resurfaces and targets Kakashi to get to Naruto, seeing their marriage as a weakness to exploit.
It could be an original character created just for the fanfiction. Maybe a mysterious ninja woman who crossed paths with Kakashi during a mission and they had a brief but intense relationship.
Often, one plot point is the initial shock of the marriage within the Naruto universe. People around them are surprised. Another is the adjustment period where Kakashi and Naruto learn to live together and understand each other's habits. For example, Naruto's messy eating habits compared to Kakashi's more refined ones.