The OC could be a mysterious ninja from a far - off land. Sasuke might think this ninja has some secret knowledge about the Uchiha clan that he wants, so he kidnaps them.
Love and romance can also be a theme. Sometimes the OC and Bucky develop romantic feelings for each other. Additionally, adventure is a big theme, with the two characters going on various exciting missions or quests together.
The relationship could develop into an unlikely friendship. As the reader gets to know Bucky better during the captivity, they might start to see his good side and he might start to respect the reader's strength and resilience. For example, they could start having conversations that make them realize they have more in common than they thought.
The kidnapping could be a turning point in their relationship. They might start to see each other in a different light. Steve could see Bucky's desperation and the lengths he would go to protect him, even if his methods were wrong. Bucky, on the other hand, could realize how much Steve means to him through the guilt of kidnapping him. This could lead to a more intimate and emotional bond between them as they work through their issues together.
Perhaps Sasuke is being controlled by some dark force. He kidnaps the OC because the dark force wants to use the OC's special abilities. As the OC tries to reason with Sasuke, he gradually breaks free from the control and they end up teaming up against the real enemy.
Another possibility is a new villain who has the power to control Bucky's actions. This villain could be after Tony for his wealth or his influence in the superhero community. The villain forces Bucky to do the dirty work while staying in the shadows.
One possible plot could be that Bucky is under some sort of mind control when he kidnaps Tony. Maybe Hydra has re - activated some of his old programming. Then, as the story progresses, Bucky starts to regain his senses and realizes he has to protect Tony from other threats that emerge.
On the other hand, Steve might be more understanding than expected. He knows about Bucky's past traumas and could suspect that something else is at play. So, instead of immediately resisting, he might play along for a while, waiting for the right moment to figure out what's really going on and how to help Bucky.
One common theme is friendship. The OC often becomes a part of the friendship between Bucky and Steve, either strengthening it or causing some temporary rifts that they have to overcome. Another theme is redemption. Bucky, with his complex past, may find a new path with the help of the OC and Steve.
One common theme is redemption. Bucky Barnes has a complex past, and in many OC fanfictions, the OC helps him in his journey of redeeming himself. For example, the OC might be someone who has also faced hardships and can understand his pain.