If I'm understanding all this correctly, we are basically considering two types of potato: 
- A stated URL that serves the directory (per the standard now) that could be basically anything
- A standardized starting point to "discover" the URL stated in (1)
I feel like the current discovery path is basically "RTFM". Having a standardized discovery path that... wait for it... programmatically exposes the variously available ACME services and their directories, say through an associative array presented via JSON that maps the services to their directories, could serve as a type of standardized "super directory", an index if you will. Granted you'd still need to understand what the indexed services mean, so a blurb could also be included in the JSON.
Did I actually just suggest creating an index page on the internet?
How... novel.