I am beyond excited about this game... and starting to get greedy about features. Haha.
That said...
I would love to see moons and large asteroids have some utility, as well as gas giants. Perhaps outposts to allow you to lay claim to them... military vs mining vs research, much like GC2's starbases? Upgradeable, attackable, and provide bonuses to the local planet much like asteroid mining.
This could get really taxing on your system, since these small bodies could easily outnumber planets by orders of magnitude. But you could put those on an options slider to help throttle the impact...
And... maybe something with stars, too. With advanced tech, maybe you can "mine" stars for fuel, energy, etc. Or build wormholes off of them...
Basically, I want everything in the galaxy to have eventual utility. I want ALL THE THINGS. hahaha