OK, I think I see your point. For example, If I want to upload video on the PeerTube network (searchable and embeddable across the fediverse via AP), 1) I need an account on at least one PeerTube instance. I can’t do it from a Mastodon instance, or using my existing account on a Mastodon instance (or GNU Social or whatever). 2) Once I set up a PeerTube account, there’s no to associate it with my existing Mastodon account. Is this it?
I don’t know enough about it to be 100% sure, but I don’t think even fully implementing the AP spec on all apps would address this. Being able to use one web ID across a wide range of sites has always been the holy grail of the social web. It’s an incredibly hard problem, because of spammers, flooders, stalkers, verbal abusers etc.
The second part is addressed by the Zot protocol used by Hubzilla. It has a feature called #NomadicIdentity. Once you have an account on a second Hubzilla instance (or “hub”), you can associated with it your first account, and then clone your “channels” (feeds/ timelines) to the second hub, and so on with as many hubs as you like. If PeerTube implemented Zot, you could associate a Hubzilla account with a PeerTube account, and clone your channels between the Hubzilla and PeerTube hubs those accounts are on, and any other PT or Hubzilla hub you have accounts on.
So far Hubzilla is the only app that implement Zot, and it’s said to be really hard to understand. But Hubzilla also speak OStatus and ActivityPub, so if you set yourself up an account on a hub, you can already talk to the rest of the fediverse from there (more or less).