Another theme could be adventure. Since Yu - Gi - Oh! has elements of adventure and dueling, the OC and Mokuba might go on adventures together, whether it's exploring a new dueling tournament or uncovering some mystery in the Yu - Gi - Oh! world. This theme allows for a lot of action and character development within the fanfiction.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can search for 'Mokuba Kaiba x OC' specifically. There are often filters to sort by popularity, rating, etc.
Archive of Our Own (AO3) is also a great place to look for kaiba x mokuba fanfiction. It has a wide variety of works, and the tagging system makes it easy to filter out exactly what you're looking for. Some stories might be more focused on their relationship development, while others could be adventure - based with their relationship as a backdrop.
One good kaiba x mokuba fanfiction could be 'The Bond Beyond Brothers'. It focuses on their strong relationship and how they support each other through various challenges in the Yu - Gi - Oh! universe.
Another one to check out is 'Kaiba's Unlikely Ally'. Here, the OC is not a typical character but rather someone with a different approach to the Duel Monsters world. Their relationship with Kaiba starts off rocky but evolves in a very believable way. It also has some great dueling scenes that are well - written and exciting.
The Blue - Eyes White Dragon could be an unexpected rescuer. In some fanfictions, the powerful card could come to life or have a spiritual connection to the Kaiba family. Also, an undercover detective who has been investigating the Kaiba family's enemies might be the one to rescue Mokuba. This detective could have been on the trail of the kidnappers for a while. And lastly, an old family friend or mentor of the Kaibas could emerge to save Mokuba. This person might have some special knowledge or skills that are crucial for the rescue operation.
First, understand the characters well. Kaiba is a complex character with a lot of pride and a drive for power. Your OC should have qualities that either complement or contrast with him. For example, if your OC is very empathetic, it can create interesting interactions with Kaiba's more cold - hearted nature.
A possible plot twist is that the kidnapper is being controlled by an otherworldly force, and Mokuba discovers a way to break the control. For example, he might find a magical item within the kidnapping location. Another plot twist could be that Mokuba's kidnapping was part of a bigger scheme to frame Seto Kaiba for a crime he didn't commit. This would add a layer of drama as Seto has to race against time to clear his name and save his brother. Also, it could be that the kidnapper is actually Mokuba's long - lost relative who wants to reconnect in a very extreme way.
Characteristics of yugi x mokuba fanfiction include their shared growth. As the story progresses, they both learn and develop as characters, often because of their relationship with each other. There's also a sense of protection. Yugi might protect Mokuba or vice versa, and this protective instinct is often explored. Moreover, the fanfictions can have a lot of cultural elements from the Yu - Gi - Oh! series, such as the importance of cards and the rules of dueling, which are intertwined with their relationship story.
Good character development for the OC is key. The OC should have a unique personality, goals, and backstory that fit within the Yu - Gi - Oh! universe. For example, if the OC is a duelist, their dueling style should be distinct.