[Z]

Or the RPG could be based on something that already exists, like a TV program or a film, and you would just continue writing stories for characters that already exist... but I prefer starting from scratch because it means anyone can take part, you don't have to have watched the TV show to know what's happening... or you could do half and half, where you take a universe (e.g. LOST) but you insert your own characters into it so you'd only need certain aspects (the island, the smoke monster, perhaps the DHARMA initiative) to carry on with it.