My attitude toward the question of established campaign worlds vs. homebrew settings has recently undergone a sea change, where running an old-school AD&D game is concerned. Well, partially at least. As a player, I'd still prefer to use the Greyhawk setting, and I still wouldn't mind running my
U-series trilogy campaign. But another campaign idea is also growing on me: running a classic megadungeon game.
"Megadungeon", as used in the OSR community, roughly means a game in the style of the original Blackmoor and Castle Greyhawk campaigns that gave birth to Dungeons & Dragons: you have a very vaguely-defined frontier region, with a huge multilevel dungeon, and a nearby city or other outpost of civilization where the PCs can resupply and sell loot. And some wilderness for variety, because while the dungeon is the main focus of the game, the players will want the occasional wilderness (or city) adventure for a change of pace.
That's all. One dungeon, one city, and some unexplored wilderness. It's deliberately a blank slate; players fill in some of the setting as they create their characters (i.e. if you're playing a cleric, there must be a church), and the rest emerges in play.
Today I sketched out a rough campaign map. The solid lines are supposed to be rivers, and the dashed line is a road. Off the map to the north, south and east are presumably more developed, civilized areas, which trade with each other using the river and road. The point where they all meet is a trade outpost which would serve as the PCs' base of operations. I envisioned it as being something like Elizabethan London, with the respectable city on one side of the river, and a poor quarter on the other.
The dungeon is up the tributary river a ways, beginning underneath a ruined castle and extending into―or rather, under―the mountains. The region west of the rivers is unexplored wilderness.
ETA: it occurs to me in retrospect that I still couldn't resist a little bit of worldbuilding, if only to satisfy my nerdish need to have things make sense. I couldn't just plunk down a city in the middle of nowhere. I had to explain why there was a city there. From which came the rivers, and then the road, and the off-map civilizations.
I guess I just can't help myself.