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.
Sokka might be a rescuer. He's Katara's brother and he would stop at nothing to save her. He could use his strategic mind and his trusty boomerang to take on the kidnappers. Another potential rescuer could be Zuko's uncle Iroh. He has great wisdom and his own firebending skills. He might be able to outwit the kidnappers and free Zuko and Katara. There's also the possibility of some unexpected characters from their past coming to their aid, like some of the Freedom Fighters they met earlier in their adventures.
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.
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.
His friends, of course. Scott would be one of the most likely rescuers. As Stiles' best friend and a werewolf, he has the strength and loyalty to track Stiles down. Lydia might also be involved. With her banshee powers, she could sense Stiles' distress and lead the others to him. Another possible rescuer could be Derek. He has a lot of experience dealing with dangerous situations and might feel a sense of obligation to save Stiles, especially if he realizes Gerard is involved.
There could also be a mysterious figure, like a vigilante type who has been following their cases and admires their work. This person might step in to rescue them, using their own unique set of skills that the kidnappers didn't anticipate.
Hermione could be a potential rescuer. In a different timeline or scenario, she might stumble upon the abandoned Harry and use her intelligence and knowledge of magic to take care of him. Another could be Hagrid. He has a big heart and a love for all things magical. He might find Harry and take him under his wing, protecting him from any harm. And Sirius Black is also a possibility. Even if he was wrongly imprisoned, if he escaped earlier, he could be the one to rescue Harry and keep him away from the Dursleys.
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.
The Akatsuki could be potential kidnappers. They are always on the lookout for powerful jinchuriki like Naruto to extract the tailed beasts from them. Orochimaru could also be a kidnapper as he is always interested in obtaining powerful bodies and Naruto's strength would be very appealing to him.
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.
It could be some rival businesspeople who want to use Mokuba as a bargaining chip against his family's company. They might be jealous of the power and success of Mokuba's family and think that by kidnapping him, they can gain an upper hand in the business world.