Space, the final frontier....
As long as it's something original in a citybuilder and not just yet another colony game like Outpost, etc. Space race, space station, but nothing too far along the purely Sci-Fi line of things either. I have in mind something more in the line of a science-possible.
OK, I was just quickly perusing this thread and didn't plan on responding or anything but this quote totally set my imagination ON FIRE!
How awesome would it be to have a city-builder that is a space station? That is, a space station-builder rather than a city. Has this ever even been done? IIRC X-Com had a pretty interesting station-building element to it, but that isn't quite the same thing as a traditional Impressions/TM city-builder in space.
You could start with different basic designs - the spinning wheel, spinning barrel etc. Then take a modular view to plugging in various "buildings" like air generation, waste recycling, residence dorms, shopping areas, and the like. All the same basic elements we get with games like the Caesars and CotNs but translated to space.
To add in the extra awesomeness and give it an Alpha Centauri feel you could customize and even "create" new modules. Maybe a hybrid residence/shopping module, a power plant that is volatile but cheaper to run, perhaps give them custom names so you can have a disco named after yourself on your station.
Perhaps take it to the next level and introduce some (very) heavily simplified 4x elements: pick a certain planet to build your station next to based on the minerals, food or whatnot you can extract from it, trade with other stations and hire out couriers and shippers to import and export the goods you trade ... set up an entire station devoted to producing water for other stations - a winner!
Thanks for this idea, it is a great one.