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.
The main antagonist could be a scientist who was responsible for the experiment that made Bucky's belly grow round. This scientist might have ulterior motives and wants to continue using Bucky for further, unethical experiments. He tries to keep Bucky under his control and stop him from finding a cure or solution to his belly growth.
One possible plot could be that Bucky, after some kind of strange experiment or accident, starts to develop a round belly. His friends are initially worried but as time passes, they notice that he also gains some unique powers related to this change. And the story could follow his journey of learning to control these new powers while dealing with his changed physique.
Sharon Carter could be in the fanfic. She could be the one who provides some medical or scientific insights into what's happening to Bucky. Maybe she has some knowledge of the experimental substances that might be related to Bucky's growing belly and tries to use her resources to help him.
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 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 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.
It could be the person or group who sent the troll. Maybe there's a mastermind behind the troll's actions, like a villain who wants to distract Bucky from something important. So while the troll is the immediate target of Bucky's punch, the real enemy is the one pulling the strings.
Maybe Bucky has to go undercover in a place where overeating is a norm as part of his mission. So, he starts to gain weight, especially in his belly area. This could lead to humorous situations like trying to fit into his old clothes or being recognized by the enemy because of his changed appearance. And in the process, he discovers new things about himself and his relationship with food.
Since I don't know the details of the 'round belly girl story', it's hard to say who the main character is. But it's likely to be the girl with the round belly herself.
It could be Bucky himself if he's under mind control. He'd be the one directly causing harm to Tony by ripping out the arc reactor, so in that sense, he's the antagonist, even if it's not really his fault.