Note that in Wikidata we are developping methods and tools to class items in « classes », which in short are sets of real world things or events. In languages like the w3c language and standards
OWL2. In this language you can assign a class to an element (a media in the common case) to a class (that can be seen as a better defined category) either by creating a statement « this media belongs to that category » (in Wikidata this is done by using
the « instance of » property) or by associating a so called «class expression» in OWL (an analog of a query but more powerful) Then in OWL any item who satisfy the criteria of the query or class expression associated to a class belongs to that class without stating it explicitely. In short, the possibility to assign an arbitrary class to an item when a query is not enough will also be possible with just a metadata repository, we may in the future even be able to mix these two ways to class medias.