@Right now extensibility of #ActivityPub shapes up as custom app-by-app app-centric development where individual devs just pragmatically throw new stuff on the wire, and when their app gains any popularity or other apps to integrate in a similarish application, things are bolted onto that in random ways. That whole story really constitutes a Big Ball of Mud anti-pattern that only introduces protocol decay, tech debt, and whack-a-mole programming, that is very hard to get rid of once there exists an installed base.
The reason that we do things that way is very understandable. It works in a grassroots environment where indivualist devs find it very hard and not valuable to collaborate at scale in what amounts to a kind of design-by-consensus process. But it comes at a high cost, where interoperability is basically out the door and any app has to be shaped as a pretzel and adopt all the quirks introduced by predecessors in a particular app domain to fit itself on the wire.