It could be a new character created specifically for the fanfiction. Maybe a young scientist who is working on a project related to the super - soldier serum. Since Bucky has a history with the serum, this would create an interesting dynamic.
Perhaps it could be another Avenger. Let's say Hawkeye. Bucky might be forced to kidnap Hawkeye due to some mind - game played by a villain. This would create a huge rift within the Avengers team and lead to a lot of drama and action as they try to rescue him.
It might be a civilian who has some sort of connection to Bucky's past life before he became the Winter Soldier. For example, a relative of an old friend or someone who was present during one of his earlier missions. This would give Bucky a personal connection to the kidnapped character and add depth to the story.
A possible plot in a Bucky Barnes kidnapped fanfiction could be that Bucky is on a mission for some shadowy organization that wants him to kidnap a person with special abilities. However, once he has the character, he starts to question the motives of the organization. He might see the innocence in the kidnapped character and decide to protect them instead. Maybe the kidnapped character has the power to heal his mental scars from his time as the Winter Soldier. So, instead of delivering the character to the organization, he goes on the run with them, leading to a thrilling adventure filled with action and emotional growth.
It could be a young adventurer. This type of character would be full of curiosity and a sense of exploration. Their kidnapping by pirates would be a major setback to their adventure plans, but they would also be quick to look for ways to turn the situation around.
It could be a young woman who is a stowaway on a ship and gets discovered and kidnapped by pirates.
It could be a young child who strays too close to the sewers where Pennywise lurks.
A young adventurer could be a great main character. They are always exploring new places, so it's likely they could end up in the wrong place at the wrong time and get kidnapped by witches.
Steve Rogers could be the one to help. Given their long - standing friendship, Steve would probably be there to offer some basic first aid or just some words of comfort.
Perhaps it's an organization that wants to use Kakashi for their own evil plans. They might think that Kakashi's skills and knowledge would be useful for them. For example, they could be trying to create some sort of super - weapon and they need Kakashi's expertise in ninjutsu.
It could be Hades. He's always been an antagonist in the series and might kidnap Pit to mess with the gods. He could be thinking that by taking Pit, he can gain an advantage in his ongoing feud with them.
It could be a rogue Saiyan who wants to take over the Saiyan throne and thinks kidnapping Goku is the first step.
It could be the Separatists. They might see Anakin as a valuable hostage to use against the Jedi and the Republic. His skills as a Jedi Knight make him a prime target for them to gain some sort of leverage in the war.