All,

Vocabularies have their own definitions already hosted.

There is no need, therefore, to replicate those same definitions in Wikidata by creating a new Wikidata item (topic / entity) for each external vocabulary class or subclass or property. 

Instead, the best practice is to simply POINT to those external definitions, such as those in Schema.org, DBPedia.org, MusicBrainz, etc., etc.

(sorry, Andra, but unfortunately, your proposal to just create Wikidata items (recreating a vocabulary inside Wikidata, instead of using Wikidata properties to point to external URLs) makes Wikidata harder to use, not easier for itself, or for external partners or vocabularies.  I won't do this, its not required, its the wrong approach, and gives grief to others that query Wikidata)

The right solution is to help propose and finish adding some of the missing 'external properties' in Wikidata, then we help the web to help us.

By adding those missing properties in Wikidata, as is done nearly everyday from what I see.  We help external communities align with Wikidata and vice-versa.

Andra - let me know once your 'external subclass' is ready for proposal review.