Now that I think on GC2 there were 2 races: Darth and Archean I think which would come handy for the size of maps we are dealing here. I remember the later as having the most interesting tech tree as they needed to construct buildings for ship speed instead of putting drives on them - I found that a brilliant idea to mix it up and it even gave the race some unique buildings.
Addition of such unique races with unique tech-trees and buildings to use also for custom races is a good value withouth meddling with the core game significantly
GC3 boosts some great animations as well for the default races - expanding the roster of leaders and active background (e.g.: Planet side palace, Starbase command HQ with planet background) in similar quality would also provide some nice DLC material
Enemy civs will be a big question for this game as is, as there are far too few colours, icons and race pictures available to play with. A lot of the colours and schemas do not give with the backgrounds so that ships often end up invisible on the strategic map.