I firmly believe creating a world for a rpg campaign is part art and part science, with a lot of inspiration necessary. A campaign world needs a lot of people (NPCs), places (maps/modules), and things (treasure,equipment, monsters).
One of the things I like about using Earth as the basis for a campaign is that I don't have to map the world! Most of that work is already done. I can use all kinds of maps that you might not normally consider a game accessory. Road maps, topographical maps, tourist maps, the National Geographic maps, the list is endless. Plenty of material to use. However, I'm always going to add something of my own creation.
One of the things I'm going to have in my Modern Horror campaign is a "lost town". Lost as in, it's populated, but no one who doesn't already live there can find it! The town is called Kinship, and early in the towns history it was cursed by Gypsies. Of course there are a few exceptions, some people can come and go as they please, but that's rare.
Sometimes people find themselves "lost", both in regards to their location, and direction in life. These are the type of people who find themselves arriving in Kinship. The town itself is home to quite a few strange people. The sheriff is a descendant of Rasputin. The town coroner is a Sorcerer. Nobody will play poker with the town's Invisible Man. And once a year the decendants of the band of Gypsies that cursed the town show up About the only thing the town's citizens don't tolerate is vampires.
The idea of Kinship came about as an idea for a novel. I still want to work on that idea, but I think it would be a great setting for a rpg. I think it would work great for GURPS or WoD.