IMHO WebDAV with a few extensions handled in the HTTP headers is what we need.

We don't need an online held-open protocol, a stateless request-response is sufficient.

The WebDAV PROPFIND/PROPATCH being in XML is dumb but usable for applying tags, and HTTP headers for certain things like 'content-type', 'range', etc, all well specified already.

We would want to be able to authenticate and to set/specify permissions (on get, put, patch, delete, but also rename, setperms, lock, delete a prop)