Aside from what I've just read on the ever-reliable Wikipedia, I'm not familiar with Bas-Lag. Am I correct in assuming the novels have fantastical, ridiculous aspects but are not zany and goofy?)
If you haven't read the books in question, you'll not really be able to get the point.
Mieville writes dark to the point of horror fantasy. I was just pointing out that a setting with cactus people, hedge hog people, caterpiller people, giant killer moths, frog folk, bird folk, and women with the heads of scarab beetles could be quite the opposite of zany and goofy.
My Gamma World games were 'gonzo' to the extent that they were being played by 14 year old boys. I never thought of them as goofy and I didn't know what campy was. I did know that a sentient cactus grabbling you, tribes of sentient social cockroaches, predatory fishes oozing radioactive polution, crazed domestic robots with household tools, clouds of flying leeches, invisible carniverous worms, or whatever else you could come up with could be pretty creepy (and cool).
I mean basically its a sci-fi setting with unleashed magic. Mutations and psionics are just magic in a dressed up form. As a fantasy setting, I don't find GW any more campy than D&D. It's a matter of how you present it. I think you could adapt the basic concept to any setting where you wanted to have a guy with a M-16 fighting alongside a guy who is basically a wizard fighting along side a guy that is basically a mutant superhero.
