The change would be using Gitmail as the plumbing, and normalising the creation of user-friendly porcelain on top.

E.g. suppose there is a repo foo/bar hosted by a forgejo instance at myinstance.org/foo/bar. Sending an email to foo.bar@myinstance.org (or similar) could automatically create a PR and, conversely, opening a PR could send a patch series to the foo/bar mailing list.