(Concluding a series of D&D campaign brainstorming based on the 1e Fiend Folio.)
One of the charming things about the 1st Edition Fiend Folio is the way that truly fearsome monsters like the iconic githyanki on the cover, or the horrid grell (a hovering, octopoid brain) share pages with misfit critters like the flail snail or the adherer (a mummy wrapped in flypaper, more or less). The 1e monster books are all much closer to the homebrewed roots of the hobby; they are less professional, less 'art-directed' both in the literal sense and in the figurative sense that they resist any overall tone or coherent vision. None more so than the Fiend Folio, much of which was drawn from fan submissions to British gaming magazine White Dwarf. The fiends within often don't make any kind of sense together, but there is a lot of freedom in the juxtaposition of the goofy and the gruesome.
Even the goofiest critters have their place in a Gamma Weird campaign: Flail away, flail snail! Glue mummies, stick around! Poisonous beetles camouflaged as treasure? Check. Mangy cadaverous skunk-weasels? Got 'em, natch. Giant mosquito lobsters? The more, and the more ridiculous, the merrier! Gamma Weird is post-apocalyptic D&D, and from where I sit the only way to play the apocalypse is for laughs.
Now, I don't mean a combination of D&D and Gamma World, a science-meets-fanstasy mix-up. I mean, that is awesome, no question. But what I have in mind is more like D&D rewritten as Gamma World:
A high fantasy realm is thrown utterly into chaos by a world-spanning magical catastrophe. Cities are destroyed by arcane forces, and the knowledge of ten thousand years is buried in the wreckage. The minds and flesh of men and beasts are warped by polymorphic energies. There are no large states, no kingdoms -- at best the land is a feudal patchwork, and most people subsist in scattered unconnected villages. Low-level spells and some simple magic weapons and devices survive, but unpredictable magical effects can make them chancy to use.
Those adventurers who seek more powerful magic will have to quest for it. Lacking a shared history and culture, social structure and mores vary wildly from town to town, so travellers may find the ways of even their neighboring villages strange. In the untracked leagues between settlements the land itself has grown eldritch and strange, rocks, rivers and hills twisted by the magical apocalypse. Creatures both comical and deadly hunt the countryside and haunt the ruined cities of the ancients, but the greatest danger the adventurers face may be the very sorceries they hope to master.