The main antagonist could be a shadowy organization that has infiltrated the Pentagon. They want to use Bucky's amnesia to their advantage, implant false memories in him, and turn him into their assassin again. They manipulate the situation within the Pentagon to keep Bucky in the dark about their true intentions and keep him under their thumb.
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.
Maybe the main antagonist is a scientist at the Pentagon. This scientist is obsessed with unlocking Bucky's memories in a way that could harm Bucky. He doesn't care about Bucky as a person but rather as an experiment, and he'll stop at nothing to get the results he wants, even if it means causing Bucky more mental distress.
One possible plot could be Bucky, in his amnesiac state, getting involved with some sort of mystery or conspiracy within the Pentagon. Maybe he stumbles upon a secret project that he has some connection to from his past. And his amnesia adds an extra layer of confusion as he tries to figure out what's going on and who he can trust.
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.
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.
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 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.
Well, it could be that Bucky has some hidden memories related to the Pentagon. Maybe there are secrets there that could clear his name or reveal some dark operations Hydra was involved in within the Pentagon. The Avengers break in to help Bucky uncover the truth. They might face various security measures and government agents who don't understand their motives at first.
It could be a fellow Avenger. Maybe Natasha Romanoff. They have a shared history and she might be someone who understands his pain and can be part of this love story.
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 about Bucky Barnes having an allergic reaction that causes him to sneeze, and his friends or teammates trying to figure out what he's allergic to. Maybe it's a comical take on his super - soldier abilities being affected by a simple sneeze.
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.