My own campaign setting is based on the premise that it was the equivalent of North America relative to the Forgotten Realms' Europe and Asia. The intent was not to fill in the map so much as to make fun of the fact that the Forgotten Realms is so transparently based on the real-world map. The continent is not called America, after the cartographer whose name appeared on early maps, but Draconia, after a cartographer's legend that read "thar be dragons." Many of the details of the setting are obscure jokes (where 'obscure' here means 'funny only to the GM') poking fun of fantasy conventions, D&D tropes, language, and American history and geography. Of course, I want to be able to run any kind of adventure, so places have been mapped out on a North America-like map for all the classic D&D realms -- the hoardlands across the great plains, a barbaric frozen north, the lands of occidental adventures out past the ersatz rockies, the underdark, the mountaintop castles of giants and the aeries of the Avariel in the great mountain chains of the east, pirate islands along the coast, crashed space ships, gaslit cities where dapper gentlemen bustle on the same streetcars with cutthroats, great railroads being stretched to cross the still not fully mapped continent, and even a dreadful isle with just about everything left out elsewhere, including dinosaurs, cannibals, amazons, volcano gods, ect.
