> "But ActivityPub is the evolution of Atom/RSS, with JSON instead of XML..."
Not exactly. ActivityPub is based on ActivityStreams. And ActivityStreams are a variant of RDF using JSON-LD instead of RDF XML. So, at best ActivityPub is based on RSS 2.0. (Which itself is overly complex IMHO.)
> "Perhaps you actually want a 'Really Simple ActivityPub'?"
Sort of. I want something much simpler anyway. But I don't want to use JSON-LD, so no direct interop will be possible.