I have been looking at planets and I have been noticing the moons. Some of them look mighty habitable.
SO, Wouldn't it be cool to have a sub planet around the actual planet. It would have about 3 habitable blocks that would add to the main planet.
Ex: Your planet (itself) has 10 income. You go to the moon and build a market center. Now the planet income is 15.
Of course there is always the issue of balancing.