I think I understand what you have in mind now 
And I agree that with federation new features will emerge. They do not exist at the moment because there is no federation between forges. But I disagree with the idea that those features should be implemented outside of a forge (FedeProxy or any third party service really). Once forges are ActivityPub enabled and use it to send/receive forgefed type messages, they will be the most natural environment to implement the plugins/components you suggest. Of course not all forge codebases will be favorable but Iām hopeful: Gitea is promising.