Perhaps Bucky kidnaps Tony because he believes Tony has information that can save his own life or help him clear his name. So, he takes Tony to a secluded location. However, during their time together, they start to understand each other better and form an unlikely alliance. Tony might even start to work on a way to fix Bucky's mental state.
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.
It could be that Bucky is forced to kidnap Tony by an outside force that is threatening Bucky's friends or something he holds dear. Once he has Tony, Tony uses his intelligence to outwit the real villains behind the kidnapping and in the process, he and Bucky become friends. This might lead to a change in the relationship between the Avengers as a whole.
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.
Perhaps Gale is being manipulated by some other force. There could be a new group or individual in the story that has convinced Gale that kidnapping Katniss is the only way to achieve a greater good, like starting a revolution. They might have fed him false information about Katniss being the key to a successful uprising.
One possible plot could be that Lotor kidnaps Lance to use him as a bargaining chip against the Paladins. He might try to force Lance's friends to give up something valuable in exchange for his safe return.
One possible plot could be Clary using her shadowhunter skills to try and escape from Jonathan's clutches. Maybe she finds a hidden weapon or a secret passage in the place where she's held captive.
One possible plot could be that the hunter is a villain from Spiderman's rogues gallery who mistakes a die - hard Spiderman fan for someone important to Spiderman. Maybe he wants to use the fan as bait to lure Spiderman into a trap. Another plot could involve the hunter being from a different dimension or world and thinking the fan has some sort of power related to Spiderman, so he kidnaps the fan to study or harness that power.
One common theme is redemption. Bucky has a complex past and Tony often has to come to terms with that. Another theme is friendship. They are two very different characters but in fanfic they often find common ground and become friends.
One possible reaction could be Tony's heartbreak. He might feel betrayed and angry, leading to a rift in his relationship with Steve. He could also become distrustful of others in the future.
Well, perhaps they meet at a Stark Industries event. Tony is the owner, so he's there, and Bucky could be there as part of a security detail or just as an attendee. They might start off with a bit of a misunderstanding, Tony being the flamboyant inventor and Bucky the more reserved soldier. But then they find common ground in their shared interest in technology, especially when it comes to Bucky's cybernetic arm. They could exchange ideas about how to improve it or just have a general conversation about the future of technology in warfare and prosthetics.
One reason could be Steve's long - standing friendship with Bucky. They have a deep history together, and in the fanfiction, that bond might be portrayed as stronger than his relationship with Tony. Maybe Steve feels a sense of loyalty to Bucky that he can't ignore.
One possible theme is friendship. Despite any past issues, Bucky choosing to stand up for Tony shows a form of friendship. Another theme could be redemption. Bucky might be trying to make up for past wrongs or misdeeds by helping Tony.