Natasha would understand Bucky's situation on a different level. Having her own dark past, she would empathize with his pain and the feeling of being used. She might offer her support in a more subtle way, like sharing her own experiences of overcoming trauma, and try to help Bucky come to terms with his past.
Thor would likely be enraged. He has a strong sense of justice, and seeing the torture Bucky endured would make him want to seek vengeance on those responsible. He might also see Bucky in a new light, as a survivor of great pain and suffering.
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.
They might be shocked at some of the dangerous situations Peter has been in without their knowledge. They could feel guilty for not being there to protect him more. And they would likely be very proud of his selfless acts.
They might be in shock. Tony has a complex past, and seeing it all unfold could leave them speechless.
The Avengers' reactions would be diverse. Captain Marvel, if she's part of the group, might be very matter - of - fact about it. Rhodey could be a bit bemused. Steve and Bucky, in the meantime, would be dealing with the embarrassment of being caught in what might be a very personal moment. The situation could also lead to some new conversations within the Avengers about personal boundaries and relationships among the team members.
Thor would be quite confused at first. Fanfiction is not something from Asgard. But once he gets the gist, he might say something like 'This is a strange Midgardian custom, but if it shows the great friendship of Steve and Bucky, then it is not all bad.' Meanwhile, Tony would be all over Steve, making snarky comments and asking if he's finally going to admit to some hidden feelings for Bucky according to the fanfiction.
In this kind of fanfiction, the Avengers might gain a deeper understanding of Peter. They could see his struggles, like dealing with school and his superhero identity. Maybe they'll be surprised by his determination and how he copes with the loss of his loved ones.
In this kind of fanfiction, the Avengers might gain a deeper understanding of Tony. They could see his past traumas, his creative processes, and the moments that shaped him into the man he is. It might lead to them having more empathy for his actions and decisions.
The characters could develop in many ways. For instance, Steve Rogers might realize he was too harsh on Tony. After seeing Tony's memories, he could become more understanding and less judgmental. Natasha Romanoff might find a new level of respect for Tony and use her skills to try and bring the team back together.
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.
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.