Part 1: Setting
Right now, it sounds kind of cliché; city-planet, alien war, fish out of water motifs and disenfranchisement. These are all kind of overdone in sci-fi, so you really have to hone down and make some choices to give your setting more life;
- If the PCs are going to be taking part as ciphers in the underclass area, what makes it unique?
- Is it overcrowded (yawn)? Or is it eerily empty, like a future Detroit?
- Will it be easier for the players to make a foothold via scavenging, or starting a trade, or trying to woo more powerful people? You don't have to discount the other options, but different cultures make different social dynamics easier.
- What's the technology level like? If the setting is post-scarcity, what economic and social factors make a person poor? As usual, look to Eclipse Phase for a well-thought out system.
Also consider the underlying philosophy of your setting; this is really where the GM has the most sway. Unlike the real world, philosophy actively affects the nature of a game's reality; certain actions are more likely to succeed than others simply because the GM can't help but have preferred methods. The point of the game is for the group to band together to survive, but the meaning of the game can strongly affect gameplay. Maybe their banding together can only minimally affect the world around them because they just don't have the options afforded to other people; the theme could morph then into "make do with what you have, because that's life" or "it's impossible to succeed if the system is corrupt, so smash the state!" Maybe the group can excel when they band together; then the tone can convey "anything's possible through the power of friendship!" (Or more likely that only the truly great can rise above their station; these are PCs after all) If you're more cynical, it can mean "if you're smart and you stick together, you can exploit everyone else in the system."
In summation: you have the beginning of a concept, but that's only the first step before you go on to building the NPCs. Figure out what you want to explore (outsiders in a strange place, the nature of a journey, etc.), through what filter you want to explore it (the philosophy), and the methods of exploration (what social dynamics are more effective).