This is something I've been thinking about blogging for some time now (since my vacation back to America last summer) and I'm finally getting around to it now. Procrastination (prepping a Chanbara adventure for my first round three play test this coming Saturday) can be a powerful tool!
Anyway, on to the content. We've all seen plenty of various campaign worlds. There are the official ones, like Greyhawk, Toril (Forgotten Realms), Krynn (Dragonlance), Athas (Dark Sun), Eberron, and of course the good old Known World/Mystara. There are plenty of worlds from other game products -- Glorantha, Harn, Golarion. There are of course alternate Earths, and worlds taken from fiction like Middle Earth, the Hyborian Age, and the lands of the Wheel of Time (all three may actually fall under "alternate Earth"). Hundreds more, not even counting worlds created by various DMs.
What's something most of them have in common? They're all scientifically plausible (if sometimes unlikely) planets.
But Ancient and Medieval thought about the nature of the world and the universe could make for some fun gaming, could it not? I don't think I've ever played in - and I'm sure I've not yet run - a Flat Earth world. And while I've used the Great Wheel (AD&D) and the Astral Bubbles (BECMI) and a few other things for planar cosmology, I've never used the idea of the Celestial Spheres. Why not?
The Celestial Spheres are maybe not such a font of awesome sauce, but it could still be cool. If you take a Dantean bent to it, it would be possible to travel on foot or by mount from sphere to sphere, rather than requiring magical means to go from plane to plane. It could allow for some lower level planar hijinks, without the gothy emo overtones of Planescape.