Well we do need a solid file format for vector rendering. I'd prefer an open one though.
Is there interest to have a map theme for http://openbusmap.org/ shipped with marble-maps? We had a Google Code-In student create it recently and the author of openbusmap.org agreed that we can use it in Marble, so it'd be easy to add it. The map theme provides a public transport view on top of OpenStreetMap.