Is there a URI or URN syntax that allows identifying particular URI schemes by their name? I could of course use an "example" URI like http://example.org for encoding the scheme in a URI, but that is not explicit in referring to the scheme and not the resource. Using only the scheme name with : also does not help much, since for example tv: alone does have a meaning in its scheme.
I am aware such URIs could be ambiguous, so something like urn:urischeme:xyz could mean different things on machines that assign arbitrary meanings to the xyz: scheme, however that is already true for any normal URI in the scheme. I am fine with urn:urischeme:xyz having precisely the meaning of the scheme of any xyz: URI, whatever that would be.
It seems uri4uri offers a sufficient way to identify any URI scheme, as well as domains, MIME types and file extensions. For example: https://w3id.org/uri4uri/scheme/urn.