My Forgotten Realms campaign, using the 3rd Edition rules, kind of evaporated many years ago due to various changes in the lives of all the players. But once in a while, I like to go back to it and see if I can resurrect it in terms of a campaign premise.
My inspiration was threefold: the new Forgotten Realms sourcebook for D&D 3E, the short stories set in Sembia as started off in The Halls of Stormweather anthology and Dragon Magazine at the time, and the Lord of the Rings (but perhaps not for the reasons you might think).
I was certainly interested in running a proper long-term campaign, and one in a setting that had a lot of pre-built baddies and well-fleshed out material (that I felt comfortable with tweaking here and there). With Faerun as the prime setting, it was a natural choice -- even if I'd shunned it back in my D&D / AD&D days.
The Sembia short stories granted an insight into what a different type of D&D adventuring might be: a city-based sort of adventuring, a politically motivated series of adventures, and the importance of trade, commerce, and power in cities that were embracing civilization in the face of existence-ending threats like liches and dragons and a thousand other entities. Also, the reference to the King in Yellow in one of the short stories made me want to introduce those Elder Gods of Lovecraftian origin into the mix.
Lord of the Rings inspired me because, among many things, it was a novel that showed off its milieu. There was a storyline that forced the viewpoint characters to leave their homes and explore quite a bit of the world that Tolkien had crafted.
Of course, I didn't want to go the route of the quest -- too stereotypical. Also I wanted PCs to be able to appear and disappear, to bring in new characters if they wished, or to retire other ones while still keeping the spine of the story line intact. What other campaign premise / frame could I use? With Sembia and the Dales as an inspiration, I decided upon a trade caravan.
Yes, a trade caravan would bring the PCs from the breadbasket of the Dales to Sembia, and through other points in between. They might encounter Zhentish opposition (or aid), if they discovered a threat to caravans in Drow-infested woodlands; they might chill at hints of foul cults praying to forgotten Elder Gods in cities; they might discover strange trends the cities that regular denizens might not, as few stray from their lands of birth...
At the same time, I wanted to recreate something I hadn't found a word for yet: sandbox play. The option for players to leave the caravan when it dissolved at its ending point, and to join another bound for another city. Or perhaps to become involved in another type of campaign premise: expeditions to hidden cities and treasure.
Alas, it was not to be. But perhaps I can resurrect some of the ideas in Mystara one of these days. There's certainly at least one caravan in Karameikos...