The best design seems kind of obvious to me. Leaving out privacy scope for the moment:
* users can create a thing -- let's call it a "channel"
* owner can choose which of their channel(s) to post to.
* Any user can subscribe to any other users' public channels
* a channel owner can delegate channel control and write-access to other users
There's room for lots of refinement... but why isn't this basic concept just always a thing?