Alternate Worldcrawl

One of the complaints against the standard D&D Planes is that, while conceptually interesting perhaps, its hard to know what to do with them as adventuring sites. One solution would be to borrow a page from science fiction and comic books and replace them with a mutliverse of alternate worlds. These would be easy to use for adventuring purposes and could put an additional genre spin on the proceedings. Here are a few examples:

Anti-World: An alignment reversed version of the campaign setting. Perhaps humanoids are in ascendance and human and demihumans are marauding killers living underground.

Dark Sun World: In this world, the setting underwent a magical cataclysm in the past and is now a desert  beneath a dying sun.

Lycanthropia: The world is cloaked in eternal night and lycanthrope has spread to most of the population.

Modern World: This version has a technology level equal to our own (or at least the 1970s) and the PCs have counterparts who play adventurers in some sort of game.

Spelljammer World: A crashed spacecraft led to a magictech revolution and space colonization.

Western World: Try a little sixguns and sorcery and replace standard setting trappings with something more like the Old West.