I have zero interest in multiplayer. Take the number of hours required to play a game of GalCiv2. Multiply that by three or more. Then try and coordinate that much time with other people. Um, no.
What I WOULD like to see (very very much and in this order) :
1. A map editor, for those of us who have zero coding ability. A comprehensive editor even, one that allows the non-coding public the ability to edit the tech tree, the races, the planet tile bonuses, the rules of engagement, and all of that.
2. More racial distinction. Race-only techs. Perhaps a race ability that makes uninhabitable planet habitable, but makes habitable planets uninhabitable. You trade racial bonuses for less competition. It could have a 10pt brother that makes all planets on the map habitable. Things that break symmetry. MOO II had dozens of these, that you could look at for ideas.
And while I'm on the subject of symmetry breakers, expand the ethical alignment bonuses and maybe add penalties for being good or evil or neutral. I love the ethical alignment thing. It adds a lot to the game.
3. Diversity of Government Types. Let me have Technocracy, with a research bonus. Let me have Fundamentalism, with a military and morale bonus. Let me have Star Fascism, with production bonuses. Let me have a government which bonuses something besides my Economy. Let me have appropriately evil governments, like Fascism.
4. How about certain weapon/defensive systems requiring certain resources before they can be built (somewhat like Civ 3.) You can then trade for these resources or find them...possibly on those useless uninhabitable planets. You could then establish outposts or mining colonies to secure those resources etc. This would provide another level of strategy.
5. More Diplomacy and Espionage. Specific Alliance types (research or economic alliances, for instance), and reasonable trading. When I have a 160% diplomacy advantage, I don't think the AI should demand all 20 techs I have that he doesn't in exchange for Enhanced Logistics. Presently it will on intelligent. Espionage with more options, like sabotage etc. Espionage tech. Espionage victories, even.
6. More jewelry and more portraits. Much more. Jewelry and ship sets for each individual race would be ideal.
7. AI improvements. Anything that makes the AI more "human" would rock. Like... taking advantage of bonus tiles.