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.
One possible plot is that the Pentagon has captured someone or something important to Bucky or the Avengers. It could be an old comrade or a powerful artifact. So, the Avengers break in to rescue or retrieve it. Bucky might be conflicted during the break - in as he has memories of being on the other side of the law, but his loyalty to his friends and the greater good drives him forward.
Perhaps Bucky discovers that there is a new threat emerging from something stored in the Pentagon. It could be a new weapon or some dangerous technology that Hydra had a hand in developing. The Avengers then break in to either secure it or destroy it. They'd have to use their unique skills. Bucky, with his combat abilities and knowledge of the place (from his time as the Winter Soldier), would lead the way.
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.
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.
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.
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.
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.
Well, in such a fanfic, Bucky could bring in new perspectives to the Avengers. He has his own set of skills and experiences from his time as the Winter Soldier. He might introduce some new combat tactics or ways of handling dangerous situations. Also, there could be a plot where Bucky is hunted by some old enemies, and the Avengers have to decide whether to protect him or not. It could lead to internal debates within the group, which would be really exciting to read about.
Perhaps the memory of Bucky's time as the Winter Soldier being forced to carry out assassinations. It would show the Avengers the extent of his victimization and how little control he had over his actions, which could lead to more empathy and understanding within the group.
One common theme is redemption. Both Bucky and Wanda have complex pasts. In fanfics, they often strive to make up for the things they've done or that have been done to them. For example, Bucky was a brainwashed assassin, and Wanda has caused destruction accidentally. So they try to find their place in the world again.
One plot could be that the cat avengers are split into two groups due to a misunderstanding. One group believes that the other is working with a group of rats who are stealing cat food. As they fight among themselves, the real villains, a gang of raccoons, start to take over the town. Eventually, the cats realize their mistake and reunite. They then use their combined powers, like the flying cat to scout the raccoons' movements, the strong cat to fight them directly, and the invisible cat to launch surprise attacks, to drive the raccoons out of the town.
The plot might involve a new villain who hacks into their chatroom. So, the Avengers have to figure out how to stop this digital threat while also communicating within the chatroom to coordinate their efforts. They could be discussing their individual strengths and how to use them against this new enemy. For example, Captain America could be leading the conversation on strategy, while Black Widow provides intel she's gathered. Hulk might be there just being Hulk, with the others trying to keep him calm and on track. And all of this happening in the unique setting of a chatroom, which adds a modern and interesting twist to the typical Avengers story.