All the ideas I've heard for this game involve it being similar to Eve. My idea is to go a different direction: make this game a simulation of starting a space colony.
When players sign up, they get to choose what colony they join. Each colony would be on some sort of planet -- maybe one would be Mars-based, one based on Venus, one Earth-like, etc. Maybe they have some sort of profession, too. The gameplay would be similar to A Tale in the Desert (note: I've never played that game, so I'm basing this on what I've heard about it). The residents of a colony will have to work together to make laws, build new buildings, run experiments, etc. All the things that would happen in a real colony. At various times they would recieve supplies from Earth, but maybe there are occasional emergencies like the supplies being lost. Maybe the endgame for a colony would be to become self-sufficient.
Here's one catch, though: In order to have new residents move in, there needs to be housing for them. So one of the decisions the residents will have to make is how much housing to build. Maybe they have enough supplies to build more housing or a new research building, but not both. And along with housing, they have to have the resouces to support the new residents (food, water, oxygen, etc). New residents won't just come from new players, though -- people will be able to transfer between colonies (maybe once a month, there'll be an opportunity to change. This way, people won't just hop around all the time). Part of the game will involve trying to attract people from other colonies, and maybe even selecting who gets to come if there are more transfers than they can accept.
Another thing the residents of a colony will have to do is justify their existence to Earth. That means making science breakthroughs, constantly expanding, showing that they're a fair society, and so on. If they can't prove that they're useful, or if one person takes over and becomes a dictator, they might find their funding dry up and all the residents will be forced to transfer.
There would be other aspects to the game as well, but this gives you all a good idea of what it would be like. I don't know if something like this would be fun to play or not, but with the number of people who like non-violent games like Second Life, there may be a niche community who would enjoy it. I know I would play it.
Thoughts, anyone?