Well they could make it simultaneously turn based.
That only fixes part of the problem, as those with bigger empires simply have more to do. You could implement a timer, but that would handicap the bigger player (though perhaps he/she should be handicapped). I've seen it done in Civ, where a single pop-up dialogue can mean you lost the battle you would've won, because the other guy got to click first.
And the only multiplayer I play is usually with family members so typically I enjoy more complex and long, drawn-out games.
This is true and also exactly the problem. The only people I'd play this game with in multiplayer are people I know very well in real life. People you can taunt when they give up before it's all over, and people you can harass when they take too damn long to finish their turns.
For multiplayer to be "useful" to the community, it'd have to have at least some success playing with strangers. GalCiv games generally last too long for that:
Many games against the AI last a good 4-5 gameyears (some much longer). That's 4 or 5 times 48 turns, so 192-240 turns for an average games. Even assuming a timer is implemented, cutting turnlength to 5 minutes (which is about what you need on the big maps), that still means games lasting 16-20 hours. Thats a long time to play with strangers, and unfeasible to play in one sitting, meaning you'd have to agree on a time and date to continue. Which the person who already knows that he's going to lose is unlikely to show up for... Even limiting turns to 2 minutes still means 6 to 8 hours of total gametime!
And then there's balance... GalCiv2 is an awesome game, but balanced it is NOT. Especially ToA. In what corner you start and what kind of planets and resources are nearby has a huge impact on your potential strength. Playing against the AI, this is no real problem, but when playing multiplayer, most people prefer the playing field fair. The amount of time it'd take to make GalCiv2 a "fair" game would be rather large, and it would problably require making the game much simpler than any of us want it to be...
I agree with you that GalCiv multiplayer would be awesome. Despite my arguments that the math siply doesn't add up, and the required overhaul to the game itself too great, I would still pay for a multiplayer expansion. I'd love to play the game with my friends, in spite of all the frustration it's going to cost waiting for that turn button to appear, etc. etc...