Lots of nice ideas here. Kudos to everyone who thought it over and responded!

Here's my two cents' worth:
1) More varied space station modules / Artificial Planets - What about modules like "Hotel / Residency", "Gambling / Resort", "Food Production", "Factory / Manufacturing", and even the ability to add engines for propulsion? What about freighters being able to ship the produced items to different planets, whether the player's or the AI's, or possibly even dedicated starbase-planet shipping routes to enable increased stats for a planet? Maybe even planet-attack weapons "a la Death Star"? In the absence of artificial planets, having more varied modules would make for a nice additional strategic aspect, enabling players to overcome the limitations of only being able to colonize a couple planets with a few squares each because other races already have 30 higher techs than the player does at game start... Of course, such bases could also come under attack....
2) A map / scenario / campaign editor / creator of some type - GalCiv2 is an awesome game (haven't gotten to the Dread Lords expansion yet), but one thing missing from any version is some kind of editor which enables us, the end users, to create our own scenarios and campaigns. The original game comes with one campaign, and the Dread Lords expansion is a second one, but other than those, we've got nothing else except the random battle map scenario creator (which is nice, but I'd like some structure, too). Something like this would increase the replay-ability exponentially.
3) Advanced Terraforming - Sometimes maps have a number of unusable planets, and it would be nice to have the ability to develop terraforming to a higher level - namely, turning those planets into habitable planets. Also, being able to terraform unusable parts of currently inhabited planets into useful squares would be nice, i.e. You've enhanced all possible squares, and still only have 3 new lands - time to begin clearing and developing new areas....
4) Enhanced War Aspects - What about fighter transport ships which are capable of transporting fighter squadrons into other sectors (much like a "space aircraft carrier", thereby allowing the player to design fighters with more firepower / armor, but less range, for more devastating attacks? Also, what about the ability to demand surrender from a planet before invading, something like "This is your last chance because a troop carrier / invasion force is sitting just outside your planet, ready to invade"...
Anyway, those are a few ideas I had so far. Lemme know what you think.