First things first: what is your story going to be about? You can pretty much pick any premise you like, as long as everyone who is going to play feels confident they know enough about the subject matter to participate in telling a story about it. You could skip over that confident amount of knowledge requirement if you prefer, but recognise that what you are playing is a pastiche or absurdist take on the premise, not a realistic portrayal. For example, a story taking place on an oil rig might be cool, but what do I know about oil rigs except what I've seen in films and TV series? Nothing really, so any story I set on an oil rig is going to be divorced from reality, which is fine so long as I don't fool myself into thinking I've understood oil rigs by telling that story.