People keep pointing out the UX fail of expecting people to have multiple accounts to use all the different fedi services. But that wouldn't be true if every #AP app and server used a general purpose #C2S API, defined in the AP spec (whether the existing one or not).
Then we could, for example, use a Mastodon account to login to a PeerTube service to browse and post videos. Or use a PT account to login to a Mastodon service to browse and post Notes.