Well, it is pretty impossible to colonize Jupiter itself. The moons are another matter. Distant Worlds does this very well, but it is more "spreadsheet" than the GC series. I haven't been able to play DW much, but it is really, really deep. You have little control over your economy as that is the private sector, but you can influence it. Every system has multiple planets with many have moons. The environments determine what species can colonize them, at least early on. It doesn't have eye candy, but it has a lot of depth.

I would like to see colonizing of moons as well in GCIII. If we can eventually have 130 factions on some gigantic map, why not moons? IMO, I would rather play a smaller map with moons (and all those factions promised on a gigantic map) on a smaller map.

Maybe that will be a DLC or expansion one day.