The basic idea is that humans live in metallic spheres extremely high in the sky. These spheres act as ships to some and homes to others. The spheres are divided into two halves- the top being the living/working area the bottom being the location of the huge engines/turbines that power the movement of the vessels. These engines are in turn powered by a white powder, called Sphax, that is created through the processing and refinement of raw clouds.
Some of these ships band together to form communities and trading posts however there are the usual pirates, scavengers etc. The sky is split into several different areas, each with it's own dangers. Some may have constant rainfall, others may be wracked by lightning. Basically different weather in different zones.
Tech level would be similar the the level during the "Golden Age of Piracy" with some slight variations to suit the theme of the RP.
Players would be RPing as new or old members of a ship, as it travels across the Seven Skies. I have a basic plot worked out but I'm happy to change it if it turns out to be rubbish.
As I said this is just the basic idea. I have a lot more developed, but I don't want to type it all up unless people are interested in the RP. If around 3-5 people are interested I'll put up an OOC with the rest of the developed idea. Let me know what you think!