skullgiver
ActivityPub uses the acct URI to kickstart itself and then uses LD-JSON to span the network. The JSON contains fields and lists that can be dynamically expanded into whatever representation you need, with default schemas ready for use through ActivityStreams.
I think it's difficult to set up a URI standard for ActivityStreams objects because there is no standard identifier, nor is there a guarantee that these identifiers will be URI-safe. Objects do contain references to (unique) URLs that identify them, but the data is linked either by value (written out completely) or as URLs.
Setting up a URI scheme can be difficult to do comprehensively. How do you represent a link to a repost of an edit of a Location object? You can't exactly expect the URL to indicate the type, so you'll probably end up with "ap:server.com/1234", but at that point you're leaving out the important part ("where do I go to fetch this object"). You can't just assume that there are standard endpoints because ActivityPub doesn't standardise any. Soms apps break on showing Lemmy content for this reason; they were written for Mastodon and Mastodon alone, so their URL generation breaks.
I think an URI scheme would just become one of those unimplemented or useless specifications. It would only distract from what I consider to be the much better solution: fixing up and implementing ActivityPub's client-server protocol.
The CS protocol lacks important things (like "how do I log in"), but it exposes ActivityPub directly. Your server will expose a bunch of lists (timelines? communities? Up to the server!) and all the app needs to do is render those. Dig down a level and you get a bunch of objects; posts, notes, comments, whatever you can think of, and they too can be rendered by the client in any way you may want.
The protocol is rather freeform but importantly, the server takes care of any references and dereferencing. Clients shouldn't need to deal with that mess of they're connected to a server that handles everything for them already.
You can write a super generic ActivityPub CS client that operates somewhat like a file browser, and then it should work with any type of ActivityPub content. A smarter app could detect the type of server responsible for managing certain things (i.e. when you're following a Lemmy community, treat posts in it as such, and not as a flat timeline), and the protocol extensions that every server adds should help with that.
The only limitation, in my opinion, is the fact that so few servers actually implement the ActivityPub CS protocol, and that in turn there are only a few applications that make use of it. I think this comes down to the vagueness of things like "how do I tell whay user this is in a standardised way" and if we can improve that part of the protocol, we may be able to get the "one single super app" for ActivityPub.