I've played GALCIV 2 and Dreadlords to death, and pre-ordered Twilight. I also just purchased SoaSE and had an unexpected surprise:
These sandbox games REALLY REALLY REALLY benefit from a HUGE acheivements list. Instead of a story, or some small trick you can build an entire empire based not on economy or military, but on beating all 8 minor races in a huge universe.
Just a thought. That achievements list in Sins is keeping me going on what would have been a very short playtime game. I think that GalCiv (which is superior in my view) could benefit even more.