I would like the actual ship models size differences between ship size classes in combat viewer to be greater than in GalCiv2. Nothing kills the epic battle atmosphere more than deploying your new massive hulled capital ship with many times the cost, firepower and crew compared to smaller size, only to find it only twice the lenght of the smaller sized hulls at most, or seeing large-hulled capship occupy the same 3D space as three-four tiny sized fighters. It really looks lame.. Tiny-sized fighers should be many-many times smaller than large or massive capships.
As for other things I would like to see in GalCiv3:
-carriers and strikecraft, Anti-SC weapons
-planetary bombardment
-capturing ships and stations
-special ship abilities, like in sins
-stargates - the galciv lore talks about them often, yet we cant build them
-minefields
-temporary cloaking for ships and anticloak technology (tiberian sun anyone?)
-asteroid fields and nebulas that have some tactical effects like in Sins (only fighters/bombers and smaller ships can navigate them - now small ships are useless once you have the bigger ones. or favoring/disadvantaging some type of weapon/defense system...)
-space creatures
-destroyable ship subsystems (weapons, engines, sensors...)