The city of Sigil from AD&D 2nd edition's Planescape Campaign setting is a great place built on the inside of a floating ring. All in all that setting was fairly strange so quite a bit could be gleaned from it most likely.
Already robbed a few bits and pieces from the setting. ;-)
I can't quite use Sigil itself however, its a little too out there.
I was thinking of using Fritz Leiber's Lankhmar, after all Fafhrd and the Gray Mouser did in fact visit the Earth at one particular point in time. It is a rather interesting fantasy location with a strong sense of "grittiness" lacking from other "famous" places like Waterdeep, The City of Greyhawk, etc (To be honest though, i'm pretty "Meh" about many of the old D&D settings, Planescape being an exception. Dark Sun being another just because it was the right kind of oddness ).
I suppose I could toss in a portal to Vornheim, but i was thinking of just cribbing the encounter tables to use in the Isle of the Unknown's unnamed Capital.
The absolute weirdest suggestion I have thus far have is to bring, of all places, Al-Amarja - yeah that Al-Amarja from the RPG Over the Edge - in all its Burrough-sean glory into the setting. Or rather have Al-Amarja = The Isle of the Unknown.
I will admit though, there is a part of me that's curious - in the same way I was curious of having "Machinations of the Space Princess" win out during Raggi's multi-kickstarter. Curious in the sense of "Could a game grounded in Fantasy/pseudo historical 16th Century "survive" a prolonged exposure to Modern/Hi-Tech beyond that of a small "blip" like the old "Expedition to the Barrier Peaks" module?"