I think some variations of that idea have been done a few times (I think there was a "map a novel" type of challenge not too long ago?), but yeah, it could work. I still don't know if I personally wanted to participate, since I have already mapped the pretty much only fantasy world that I love enough to bother (Le Guin's Earthsea), but it might be a bit more inclusive to non-GOT-fans and less specific, as Falc said.