I'm not sure if development is too far along to implement this, but I was thinking that perhaps moons would be an interesting dynamic to add to the game. They would not only extend the gravity well of a planet, but also, when colonized, would provide a fractional bonus to the planet's tax income. It would also be interesting to see if multiple players could colonize a planet and its moons, sharing a gravity well. Perhaps the owners of the moons could pay tribute to the planet's owner as long as they were neutral or friendly to that player.

Gas planets would go hand in hand with this. Though uninhabitible by themselves, they would likely have an enormous amount of moons.

Just some ideas. Feel free to offer your own feedback and expand upon them.