I'd suggest something straightforward to start with that everyone could jump into. Since you propose something about Saturn's rings, maybe lean on Heinlein's stuff for inspiration (and, of course, the original "G" of space westerns, Firefly):
Setting would be the solar system, almost fully colonized by humanity. The Inner Planets are under the control of the United Terran <insert synonym for federation/syndicate/whatever>, the futuristic descendant of the United Nations. The Outer Planets, while nominally also under UT- control, are far enough away from central authority that local governors/ruling councils tend to hold absolute authority over their particular domains and piracy is a major security risk. Even though the world nations all answer to the UT-, they still independently maintain and operate space programs, leading to an awful lot of tension and proxy skirmishes between some of the dominant powers. Until this is resolved, the colonial areas on the outer planets will consistently have to deal with their own problems with minimal support from Earth, Mars, and Venus.
Colonies consist of domed cities on most of the gas giant's larger moons, with Titan having been completely terraformed into a slightly smaller Earth-like planetoid. Massive free-floating starbases (think Deep Space 9) are also scattered through the moons. An expedition (colony liveships, terraforming rigs, and equipment/supply freighters) set out for the illusive ninth planet
, Nemesis, with the intent of establishing a colony, but humanity lost contact with Expeditionary Force 141 a month after departure and has not heard from them since.
At the time the RP starts, the piracy rates have begun to rise, with many pirate vessels now displaying the same emblem of unknown origins. A bizarre burst transmission was picked up by the Pluto outpost, a transmission that partially matches the distress signal encryption assigned to EF 141. The player characters have all gathered at <insert city name>, the capital of Titan, to enter for a chance to be part of a rescue mission tasked with tracking down the source of the mysterious transmission and attempting to discern the final fate of EF 141.