(the way remove-all-our-cc-data-and-create-them-again-under-GPL is out of the question)
Why not? We've created it once, we can create it again.
Seriously though, it would require asking every single person that contributed under any license than the one we want to keep (and compatibles) to double-license his works. It can be done, e.g. people did it with the games Moria and Angband.
I'm hope I'm mistaken, but I'm afraid, we have no choice. Even if there are no maps with textures and in-map models under incompatible licenses, even if the whole game is not legally a derived work of all of its artistic components, there will still happen such cases as a screenshot on which you see a map under CC-by-sa, fonts under GPL and aliens under CC-by-nc or an in-game video with a soundtrack mixing elements under incompatible licenses. The person that publishes such a screenshot or broadcasts such a soundtrack is then violating the licenses...
I guess what we can get away with is a separate licence for the code (GNU GPL), another one for sounds _and_ music and another one for the rest (visual and textual content together --- they are sometimes displayed on the same screen). But we would have to ask for advice some specialized lawyers (FSF? CC?).