@js It's entirely possible to build on top of the core ActivityPub API (CRUD + follow + collection management). I think it's a great thing to work on.
I think it's a good idea to take it piece by piece. I've done some work on this with the `blocked` collection FEP, `pendingFollowers`, and so on. We need to keep that momentum going.
We can even merge popular extensions into the context file for Activity Streams 2.0 so they become indistinguishable from core.