The usage note of dcat:themeTaxonomy says "It is recommended that the taxonomy is organized in a skos:ConceptScheme, skos:Collection, owl:Ontology or similar, which allows each member to be denoted by an IRI and published as Linked Data.".
But the range of dcat:theme is skos:Concept, so it seems that the expectation for the members of the objects of dcat:themeTaxonomy statements is a bit more precise than what the usage note says.

Future work?