Who could be the antagonist in a 'Winter Soldier kidnaps Tony' fanfiction?

One possible plot could be that the Winter Soldier is under some mind control still and is made to believe Tony is a threat to a certain organization. So, he kidnaps Tony to bring him to his handlers. But during the journey, some of the Winter Soldier's old memories start to surface, which makes him question his mission.

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.

It could be another dark guild. They might be using Gray as a pawn to kidnap Lucy, perhaps because they know of her celestial spirit magic and want to use it for their own evil purposes. Gray may not be fully aware of their manipulation.

It could be Aizawa himself if he's being controlled by something or has some sort of ulterior motive that is not in Izuku's best interest. But if not Aizawa, then there could be a hidden villain who is manipulating Aizawa to kidnap Izuku.

It could be a new, powerful dark wizard who has a grudge against Fairy Tail. This wizard might manipulate Natsu into kidnapping Lucy to create chaos within the guild. The dark wizard could be after some powerful magic that only Lucy can access, and by kidnapping her, they hope to force her to give it up.

Well, Vegeta himself could be seen as the antagonist in the beginning. Since he is the one doing the kidnapping, he's causing distress to Bulma. However, as the story progresses, the real antagonist could turn out to be someone else. For example, another alien race that is after Vegeta and uses Bulma as bait to lure him into a trap. So, it's not so straightforward in a fanfiction.

It could be Orochimaru. He might have manipulated Kushina into kidnapping Naruto. Orochimaru has always been interested in the power of the Uzumaki clan and Naruto's nine - tailed fox power. He could be using Kushina as a tool to get to Naruto.

One great fanfiction could be where the Winter Soldier and Tony Stark are forced to team up. In this story, their initial distrust gradually turns into respect as they face a common enemy. They learn from each other's skills, with Tony's tech knowledge and the Winter Soldier's combat abilities complementing each other.

There are probably some. Fanfiction writers often explore different relationships between characters. You might find some really interesting stories with this pairing. Some could be about their unexpected friendship, or even a more complex emotional bond in an alternate universe scenario.

There's also 'Bucky's Vow'. In this fanfiction, Bucky feels a strong sense of protectiveness towards Tony from the start. It delves deep into their characters, showing how Bucky overcomes his past traumas to be there for Tony. The chemistry between them is really well - written.