last post I started a thread that I think interests me most in the higher planes: exploring their use as literal planets, and bringing out the idea of using the Great Moon Hoax of 1835 as a basis for lunar adventure - which, the more I think about it, the more I want to write as a module. But the lunar bat-men (referred to in the hoax as vespertiliones-homines, to use the Latin plural) are explicitly peaceful surface dwellers, which leads me to H.G. Wells's classic The First Men on the Moon, which helpfully has a race of more hostile insectoid men called "Selenites," who live in a complex underground civilization and tend giant mooncalves. So there is your conflict: there are peaceful bat-men and terrifying insect-men, and it even lets us have a dungeon under the lunar surface.
I imagine a module for this will be done before the bestiary, since I have less than a dozen monsters actually complete and need several hundred, while the module's requirements are much slighter. This also fits perfectly into my concept of the module as an adventure that can be fitted easily into an existing tentpole dungeon or sandbox, as Matthew Finch did in Demonspore.
The rest of the planes in my vision are similarly planetary, becoming more rarefied and "pure" in their approach to law and chaos as they go, eventually reaching the godly realms outside of the celestial spheres themselves. I see Yuggoth as being one of those planets, leaning heavily towards chaos, and Jupiter and its moons being the much more Lawful versions.
Barsoom. The original D&D booklets made direct reference to creatures of ERB's Mars, and TSR briefly published a game (now noted for its rarity) called Warriors of Mars. To say it is ripe for gaming is practically a truism; Barsoom was reached from within Castle Greyhawk itself, giving it the key D&D imprimatur.
White apes, green martians, banths, calots, thoats - these seem like they should be as basic to D&D as the goblins, orcs and so on of Tolkien's world, and red Martians as familiar as elves. They give it a much more "alien" feel that I think many DMs are looking for, and are at the same time instantly recognizable for most gamers. I know there is a need to be careful around the IP waters, but I can't help think that the bestiary would be much richer if it included creatures from the public domain Mars novels.
Barsoom is also a great setting for the Law/Chaos dynamic; when John Carter encounters it, it has clearly gone toward Chaos, with great civilizations having decayed into war and barbarism without end. The efforts of Dejah Thoris and John Carter to restore that civilization is an effort to bring back the cosmic balance that has been upset in the world.
Next entry (either tomorrow or Sunday): Jupiter and Yuggoth. After that - Hell.