We seem to be agreed on making this a post-apocalyptic game, in the vein of Elite, giving us and (more importantly) the players a great deal of flexibility about what can be done. In order to put some more constraints on thing, I'm quite keen on the idea of not only having inhospitable areas, but setting the whole thing in desert conditions. This has the potential advantage of actually forcing the players to do things, since if they sit around doing nothing, we can make them use up their water supplies, or something.
What we haven't agreed on as yet, is whether we're looking at an Earth (or near Earth) analogue, or something totally alien or fantastic. I'm inclined towards avoiding classic fantasy, but going for a near Earth analogue.