Agreed. A friend of mine I won't name has been working on a novel that became a story arc that became a series of novels that became a universe, none of which have been released. Creating an imaginary world is certainly a good idea, but 99% of the time it does bugger all for the quality of the novel - usually making it worse, as the story is continually disrupted to retell backstory upon backstory. Unfortunately, I think that this has created a stereotypical idea of fantasy novels, which means the good stories don't get the attention they deserve (unless they are made into hit TV shows). I like the mythology angle that you presented. Series like Discworld take another good approach - very complex mythical world, but it feels like it grew organically as the series progressed, it doesn't feel like it was all planned top-down when the series started. Places that were just referred to in one-line jokes were later fleshed out as Pratchett pleased.