The DCAT profile mentions Eurovoc as proposed vocabulary, which is an excellent suggestion.
However, the SKOS/XML version of Eurovoc still seems to require that a (free) license is obtained from the Publication Office. It would be nice if this licensing step is removed, and Eurovoc simply becomes available as open data under a Creative Commons License.
(IIRC e.g. the European environment thesaurus GEMET is available under CC-BY license)