Another idea for an RP server. Two or three cities, competing for dominance over the landscape. The first city I built (and really, the only one) was inside of a mountain. Still expanding it. Various guilds would control various aspects of the city's economy and everyday life, so it could have hypothetically supported a large number of players in a non-peon role. There's a lot of little nooks and crannies- almost all the space in the mountain is used, so explore it!