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.
There could be a new super - villain who has infiltrated the Pentagon and is using its secrets and technology for their own gain. This villain sees the Avengers' break - in as an opportunity to take them on and prove their superiority, especially targeting Bucky because of his reputation and power.
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 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.
In a fanfic where the Avengers break into the Pentagon, it could be for a number of reasons. Maybe they suspect the Pentagon is hiding some dangerous alien technology that could fall into the wrong hands. They might use their unique skills to bypass security. For example, Black Widow could use her espionage skills to disable alarms, while Iron Man could hack into the security systems.
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.
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 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.
They could be after a specific person in the Pentagon. Maybe a rogue agent who has stolen some important data related to the Avengers and is planning to sell it. So, they break in to stop this from happening.
Loki could be a great main antagonist. He could be the mischievous student who's always causing trouble for the Avengers. He might use his magic to play pranks on them or steal their homework to make them look bad in front of the teachers.
A new, original villain created just for this fanfic could be the main antagonist. This villain could be a powerful sorcerer who has been observing the Avengers from the shadows. When the OC is reborn, they sense a disruption in the magical balance of the world. Believing the OC to be a threat to their own plans of ruling the world through magic, they set out to eliminate the OC and in the process, come into conflict with the Avengers.
Thanos could still be a great antagonist even in a fanfic. He could come back with a new plan to obtain the Infinity Stones again. His powerful and menacing presence would create a high - stakes situation for Captain Marvel and the Avengers to face.