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 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.
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 corrupted version of one of the Fairy Tail members from the other dimension. For example, an evil Natsu who uses his fire magic for destruction and domination. The real Natsu would have to face his own dark self and find a way to defeat him without losing his own identity.
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.
It could be Tony. He's always curious and out - going. He might have noticed Bucky at an event or something and decided to approach him just out of sheer interest.
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.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
The main antagonist could be Shere Khan. He has always been a threat to Mowgli and might see kidnapping him as a way to finally get rid of the 'man - cub'. He could be working with some of his sly jackal minions to carry out the kidnapping.