I imagine ability to fork, comment, open an issue or a merge (pull) request, do a code review etc from an account on one instance to a project on another. That would enable true decentralisation of software development. It was one of the original promises of Git, but was lost with the emergence of GitHub. With such federated network of forges each developer, or a group working on a project, could run their own server and collaborate with anyone else, without registering accounts on hundreds of services. I’d love that.