Didn't I hear somewhere that some kind of defensive battle-station is already planned for a future patch/expansion? I'll have to look that up.
I don't think it was ever said that stations are "planned", but there was a mention of a possibility

As for moons, they've been suggested since beta. Still, for realism's sake, the game does have them. The Earth's moon is pretty unique as far as moons go, and most are just pieces of rock trapped in a planet's gravity. Take a look at Jupiter, with its 60+ 'moons', most of which are just chunks of rock. In that sense, these are already in-game. Most grav wells have rocks if you zoom in close enough, and the resource asteroids are essentially moons as well.
It's not quite the common idea of a moon you can interact with, but nonetheless