The good news is that there will be a Galactic Civilizations III. The bad news is that it won't be out this decade.
Galactic Civilizations II v2.0 is currently in development but it's free for all players of the expansion packs. But that will serve as the code basis for any further GalCiv II updates.
Right now, the team is working on the "unnamed fantasy strategy game" sometimes called "not-MOM" (not Master of Magic). It's a totally new graphics engine that makes use of multi-core CPUs and GPUs but will still run fine on lower end hardware thanks to built-in detection that will determine "how much stuff" to display in real-time.
That game will go into public beta early next year and its release date will be largely based on player feedback. As many of you know, we are in the position of being able to keep working on our games until everyone's happy with them. Our non-game part of the company does so well that there's no pressure. We want to make it the best turn-based strategy game of all time.
THAT engine will be what serves as the basis of a future Galactic Civilizations III. That means GalCiv III will have features like tactical battles (as an option), multiplayer, more sophisticated planetary development, and much more.