Well, obviously the Avengers themselves are the main characters affected. Captain America would be one of the most affected. Given their long - standing friendship, any shock from Bucky would deeply impact Steve Rogers. Tony Stark would also be greatly affected. Their relationship has been rocky in the past, and if Bucky does something unexpected, it would likely stir up new tensions or alliances between them.
In this fanfic plot, Bucky's character development could be significant. His family's love and acceptance could give him the confidence to face his past traumas. He might start to heal from his inner wounds. For example, if he used to be withdrawn, he could become more social within the family. He could also learn to trust again, which is a big step for him considering his experiences. He'll be surrounded by people who care about him, and that positive environment will help shape him into a more positive and stable person.