A friend of mine set a Fantasy Hero campaign in a world that had an ancient semi-abandoned city suddenly appear in the middle of a wasteland. In a chamber in the palace was a huge man who seemed to be in some sort of stasis. In another location was a fortress where time seemed to flow at different rates in different rooms. There was also a city similar to Sharn in the Eberron setting, but this was several years before Eberron was published. When the PCs at one point went to visit the elves they learned that the elven goddess resided inside an enormous ancient tree, and if the tree died magic would be lost to the world.
Another friend developed a campaign set in a vast Byzantine city, so huge that most of the residents had little or no concept of the world beyond their city. You could travel for days and never leave its confines. In one area an enormous rift ran through this metropolis. Our entire campaign never left the city; in fact most of it never left just one district of the city. We just called it the City, as if it was the only one in the world (although one of the PCs did come from someplace outside the City). It reminded me a little of the city of Nessus in Gene Wolfe's Urth of the New Sun books, always growing and leaving dead parts of itself behind as people abandoned the most distant areas to move toward the newer areas.
I was also briefly involved in another Fantasy Hero game set on a flat world. At one point the PCs went over the edge of the world and had an adventure on the other side, which was covered by a vast ocean.