I got this idea for a story: a very small planet, the size of a very large city. The core of the planet would have to be very dense, to have enough gravity to make the residents keep their feet on the ground. You could walk in any of the four cardinal directions from your house and arrive at your house again in X number of hours. I guess the metaphorical implications are obvious, the "small planet" and all. You could also see it in Star Trek terms, where they always seems to land in the capital city of any given planet, and everyone speaks English.