This is something that the new eye candy got me thinking about. Colonizing Satellites. It would seem to me that after a planet is fully developed, a reasonably smart race would look at all that airless, low gravity real estate in orbit and think, "Why not?" Nothing spectacular, maybe not even developable real estate like a planet, but a single development like "Satellite Research Colony" giving additional research. "Satellite Manufacturing Facility" for Social/Military production, or "Satellite Colony" for more population (and the associated income increase as well).
Maybe have each type of colony unlockable under a different tree.