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 also be a new villain who has somehow managed to hack into the Winter Soldier's programming and redirect his mission towards kidnapping Tony. This villain might be someone who wants to create chaos between the Avengers by pitting the Winter Soldier against Tony. They stay in the shadows, pulling the strings while the Winter Soldier and Tony are caught in the middle.
Itachi himself could be seen as the main antagonist in this scenario. After all, he is the one doing the kidnapping.
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 a corrupt general within the Pentagon. This general might see Bucky as a tool to further his own power and is willing to do whatever it takes to keep Bucky under his control, especially since Bucky has his amnesia and is in a vulnerable state.
It could be a jealous rival of Bucky. Maybe this rival is envious of Bucky's popularity and when he sees Bucky's round belly, he uses it as an opportunity to mock and undermine him at every turn, becoming the main source of conflict in the fanfic.
It could be a high - ranking military official who believes the Avengers are a threat and is determined to stop them at all costs. This person might see Bucky as a dangerous weapon and want to capture or eliminate him.
Maybe a new super - powered villain who has the ability to manipulate minds. They target the meeting because they want to turn the superheroes against each other. Since Bucky has a history of being mind - controlled, he would be especially vulnerable to this villain's powers. So, this villain could be a great antagonist in the fanfiction. They could start by subtly influencing the conversations at the meeting and then escalate to full - on attacks.
Maybe the main antagonist is an ancient evil that has awakened. It has the power to manipulate Artemis's mind, making her kidnap Percy against her will. This evil entity wants to use Percy as a sacrifice to gain more power. So, while Artemis is the one who physically kidnaps Percy, the real force behind it, the ancient evil, is the true antagonist.
A powerful and corrupt organization might be the main antagonist. They blackmail Wade into kidnapping Peter. The organization wants to study Peter or use him as a bargaining chip. Wade is caught in a difficult situation where he has to protect Peter from this organization while also dealing with the fact that he has kidnapped him.
The main antagonist could be the head of the CIA operation that orders Link's kidnapping. This person might be power - hungry and believe that Link can be used to gain more control or knowledge.