An idea I've had for a long time and that I've never yet had the chance to fully implement is the creation of an "interplanetary" D&D setting, by which I mean one in which there are multiple inhabited worlds, each of which is (or has been) in contact with the others. This strikes me as true to the pulp fantasy inspirations of OD&D, with its regular invocations of Burroughs, and also with Vance's The Dying Earth, where there are frequent references to powerful wizards traveling to other moons and planets. I've also felt that the existence of congress with other worlds made it easier to justify the smörgåsbord of weird creatures that inhabit most D&D worlds. If I can say, for example, that lizard men are natives of the Green Planet and that those on the main campaign world are degenerate descendants of interplanetary travelers from the past, in my mind it's all a lot more "believable," if that makes any sense.
In my mind, I always imagined that the main campaign world had two sister planets, which I called simply the Green Planet and the Red Planet. I might give them actual names, but, as I envisioned it, regular travel between worlds was no longer common, having been curtailed by some cataclysmic event in the past (because all good fantasy worlds need a cataclysm in the past), so those names were known primarily to sages and savants and the rare wizard who still traveled there by arcane means.
The Green Planet is pulp Venus -- a lush, steaming jungle world filled with intelligent apes, all manner of reptiles (including dinosaurs), and weird amphibians. Imagine the Isle of Dread as an entire planet and you'll get the idea. The Red Planet, on the other hand, is pulp Mars -- a parched desert world whose decadent civilization teeters on the brink of collapse after untold millennia of practicing black magic and demon worship. Imagine if H.P. Lovecraft had written A Princess of Mars and you'll have the right of it.
What's fascinating is that Paizo's Pathfinder RPG setting, Golarion, seems to be trying to mine the same vein of pulp science fiction. It even uses the terms Green Planet and Red Planet much as I do. It's stuff like this that makes it hard for me not to wish the guys and gals at Paizo well, even if their taste in game mechanics are decidedly more new school than I like. If I ever get to use this in a campaign, you can be sure I'll be swiping a couple of ideas from Pathfinder.
