Over at From the Sorceror's Skull, Trey has been posting some weekly blurbs he's calling "Apocalypse Under Ground" - readers here should them check out. (And, uh, Trey should label them or something so I can link to the series instead of the first post…) The central conceit is this… what if D&D's "adventurer culture" was a recent thing? What if some fundamental thing changed in the near past, and suddenly dragons and monsters and dungeons and nightmares of every shape and form start coming back to the surface from deep underground?
Take your favorite myth cycle or bits of legendry and develop a suitable apocalypse - perhaps it's the beginning of Ragnarok, and the barriers separating the Nine Worlds have faded. The island of Avalon has returned, the Greater Seal of Solomon was shattered, or it's the time of the astral conjunction or harmonic convergence. Maybe something has shifted in the cosmic battle between Law and Chaos, and the intersection of the mortal world with the magic realm is the result of Chaos triumphant. Like the picture above, showing Oberon and Titania* re-entering the world, I would use the return of the various Fey as the initiator of "The Change".
Here's what I like about this type of campaign setting for D&D… you can take your favorite real world historical period, dump all this magic and chaos on right top of it, advance the timeline a few years, and then roll in with the adventurers. It's the Thirty Years war, and the world groans beneath the weight of religious strife and constant warfare, until the horror is complete and the doorways to chaotic Faerie gape open again, allowing goblins to swarm across the battlefields of Europe. Trade and travel suffers from a countryside suddenly overrun with monsters from beyond. Where there was a barren moor, a strange tower has appeared overnight; the dead rise from unhallowed earth, and in the heart of every kingdom, at least one yawning cavern grants access to the dark places underground where the monsters have waited, and watched. The only place where people can find the kinds of weapons that can be used against the monsters is by entering the dungeons.
The problem with using the real world as a D&D setting is rewriting history to account for the presence of humanoids, demi humans, and magic, and by the time you shoehorn them into this faux history, you've pretty much rewritten The Grand History of the Realms or The Silmarillion. I'm pretty lazy. I'd want to use the real world as is, keep a few thousand years of world history the same, up until about 5 years ago when everything changed during the Chaos Apocalypse. All this magic shit happened and dungeons and the Underdark appeared whole-cloth overnight. All those things whispered about in dark fairy tales and hinted at in ghost stories are suddenly true, and they want to eat you.
I'm filing this one in The Junkyard** to refer back to on a rainy day.
Edit: Trey is kindly labeling the series, you can find all the entries here: Apocalypse Under Ground
*Images: The top one is Charles Vess from, Sandman: A Midsummer Night's Dream, the bottom is the cover to Uncanny X-Men #190.
**The Junkyard is the place I put campaign ideas that I'm just not ready to act on; it helps me manage my gamer attention deficit disorder by putting future campaign notes somewhere for reference and gets them out of my head for a bit. Next time there's a TPK or we're ready for a change, I know I can visit the Junkyard and see what kind of ideas are stored away gathering dust.