GitHub Webhooks  allow to build for repositories custom tooling and integrations. Currently adding new webhooks requires involvement of repo admins and adds a [relatively small] maintenance burden to keep the list of webhooks up to date.
I propose to add some publish-subscribe mechanism (like [2, 3, 4]) to make it easier to develop new integrations and to maintain existing ones. The main goal is to see if the community is interested in the idea, not to discuss specific services that can be used for this purpose. Just want to note that it is fairly easy to accomplish and cheap to keep running.