Developed?
No.
Jotted down 100+ pages of random, disjointed notes that never resulted in anything concrete?
Oh yes!
The basic premise was that the original Earth was destroyed, but the Moon survived-- as did all the gods who live on it. So the setting pantheon consisted entirely of moon and moon-related gods from all various cultures of Earth, divided up into three "Houses". These gods started off mostly drained of power (all their worshippers have been vaporized, after all), but slowly reacquire it as the New Earth recondenses from its ashes then evolves; and also as the moon gods create new lunar beings in their own images, on their own turf.
I was intending to to run it as a series of five or six short, sequential campaigns, where the "tech level" and cultures played on the Moon are sort of "an era" ahead of whatever is happening on Earth. So, for example, the first campaign is lunar "Stone Age" while the Earth is still a molten ball of rock. When Earth progresses to stone age prehistory, the Moon has moved on to "ancient civilizations" comparable to Egypt and Ur. And so forth, through several steps up to "modern" Earth, at which time the Moon has collapsed into a subterranean, post-apoc nightmare world (i envision a cross between John Carter and Aliens). There was one last future era, inspired by LeGuin's story "Vaster than Empires and More Slow," in which the Moon and Earth had become joined together in a single planetary-scale knot of biological/vegetal tissue. But that was getting a little "out there", heh.
3e was the thing when I started thinking about this. The world incorporated a lot of the standard D&D races and monsters, often reskinned a bit or outfitted with templates. Doppelgangers, sphinxes, certain celestials/infernals, and phase spiders figured especially heavily, too, because reasons i've forgotten.
There's a lot more to it, but that's a capsule overview.
