I would not presume that the lack of any public roadmaps or announced funding rounds for C2S projects is a reliable indicator of nothing happening outside of the Mastodon API. In fact, a’s very subtle point about “AP objects” being an oxymoron and all objects that AP passes around actually being “AS2 objects” might be less academic than we know! I wouldn’t build a C2S project without some real funding, and I can’t imagine many funders of such a project wanting its fundees to blab about such a project before launch 
To put it another way, a microblogging project whose northstar is interop with MAUs would, of course, be crazy not to measure all design decisions against S2S legibility to the users on Threads and Mastodon. But other kinds of projects would naturally have other metrics…
I love this line of questioning, and I’m not sure that discussion needs to happen elsewhere! One nuance of our kooky JSON-LD ontology (actually defined in RDF which is ignored by people working in JSON-only mode) was totally lost on me until last week when a pointed it out to me in another thread: any JSON[-LD] Activity has an “implicit” attributedTo by dint of having an actor property, at least when expanded to RDF, thanks to the ontological definition of Actor. Might (or should) inboxes and outboxes have an “implicit” attributedTo as well, i.e. the Actor object they sit in? If actors and activities both have attributedTo properties implicitly, making them mandatory for Objects might not be as radical an extension to the protocol. Indeed, it might be a simple and elegant way of figuring out a lot of nagging interop problems like CORS/cross-domain ownership, group actors, “Server Actors”, etc. I’m all for it!
Selfishly, I’m poking this hornet’s nest on purpose to get smarter people’s perspectives on what might, might not, should, and should not break when an Actor object sits in a different domain than its Inbox, in general, not just in the context of this FEP. I feel like so much of the nuance that went into the design of the AP protocol (often implicitly or contentiously) was lost in the square-peg/round-hole of the spec’s editorial process… and now needs to be written down urgently by different people intuiting and reverse-engineering it. If nothing else, debating these hunches and hypotheses here helps us establish a baseline understanding of status quo against which to debate breaking changes, and also to settle “who’s fault is this” interoperability disputes 