If you look at this post on the development of Galactic Civilizations over the years you can see a list of features that are going to be in Galactic Civilizations III. If you've played the Alpha, you'll notice a lot of things on that list aren't in the game yet:
United Planets, Galactic Resources, Galactic Events, Ship Design, Combat Viewer, Cultural Conquest, Asteroid fields*, Invasion tactics, Fleet combat decisions, nebula and ion storms*, strategic resources, and probably a few more that I've omitted because I'm not sure yet if they're in the alpha.
*Asteroid fields and nebulas/ion storms are in the alpha, but don't currently do anything as far as I know.
EDIT: thinking about it, nebulas and ion storms do have effects on the gameplay, but for some reason I thought they were supposed to have resources as well, but I'm unsure about that.