One longer-running campaign I ran evolved organically over time. I’ve revisited it a few times with an eye toward running it again or, at the least, culling some of the better (and less derivative) ideas that grew out of it.
The campaign began in the titular Glen with the well-worn idea that the location would serve as the home base and jumping off point for a series of adventures, much same way as TSR’s village of Hommlet. The Glen marked the northernmost outpost of a large unnamed kingdom. From there, the player characters would initially investigate rumors of a possible goblin threat that would turn out to be the harbinger of an all-out invasion. The visual inspiration for that initial foray came from Jeff Easley’s cover art for the Monster Manual II and the idea of a monster-haunted forest (much like Mirkwood as Iron Crown’s MERP setting was an influence, but rather more deciduous and slightly less dangerous à la the Trollshaws).
As I added elements to the game and the threats escalated to match the level of the characters, the Glen was quickly left behind in favor of an extended foray forever deeper into the lands where evil forces had gained a far greater foothold than anyone imagined. The idea of locations linked by a larger story arc obviously drew in great deal from the Giant and Drow series, but taking place almost entirely overland and sharing far more with the Dragonlance series. I was also inspired by the military campaign style of Glen Cook’s Black Company series that first came out at that time.