first of all excuse my english!
Here are some ideas that for any GalCiv sequal/expansion pack that i think are unique and have not been posted on the forums.
Vassal States
Making it possible to allow minor races to join your empire and still have independence. Only minor civs could become vassals, through capitalation? or through peacfull means (they ask to join without war). With this you would have access to unique diplomacy with them (not sure what but you get the picture). Also you can give them back independence after 2 years or something. You could mingle for hours with this feature.
Unique Weapons/Buildings/Hulls/Defense
It would be cool to see every race, with unique buildings (only one a planet) and weapons and STUFF! like the Torians having a cloning sector which increases population growth, or the drengin with brown noise phasors! etc. Each would have to be at the end of the approriate tech branch and it would have to keep the game balanced!
Now some ideas i have seen that i agree with
Mega Events
They would be so cool! having a race from another galaxy threaten all lifeb within ours and we have to unite and defeat them! The possibilities are endless with this feature.
Artificual WormHoles
Like starbases that are like stargates! you can dial another gate built somewhere and send ships through! To keep it balanced though i don't think it would be added to the game! But you could ballance it!
Forgive me
Carriers
Yes thats right, BUT i think they shouldn't allow you to build small and tiny vessels and load them on! TAHT WOULD BE WRONG it could be a defense module, OR COULD JUST BE THERE TO MAKE THE FLEET BATTLES LOOK COOLER
I have got more but i don't think people are going to read it all!