Since the topic "combat improvement" has already been discussed (my 2 cents, a tactical mode would be too beautiful but also too difficult to implement), I'll go with my wishlist for another topic that I really enjoyed in GalCiv3, the Ship Designer:
1) first, standardize the textures among the various shipsets; it was a real pain to find the perfect part for the ship you were building and being unable to use it because its textures made it stick out from the rest of the hull like a punch in the face;
2) add more single color parts. Add also plain forms (plain cubes, cilinders, pyramids and so on) which can easily be mixed with the other parts to obtain particular shapes;
3) aside from simple strips, there are really few light parts, so you should add more with various shapes (like a ring of light that can be use for hypergates or the spheres of light used for GalCiv3 Relics);
4) a big improvement could be the possibility to add pauses during the animations of a piece (for example, making a piece turning to the right 30°, then pause for a second, then turning back to the left).
So far, these are the first things that come to my mind, maybe later I'll find something else.
However, speaking of combat, there's something I want to say: if I got right the "Map of maps" feature, the new game map will see the various star systems grouped into star sectors, which will be linked by star lanes for travel. If so, this would give a new purpose to the Military Starbases, as you can build them at the end of a star lane to create a choke point if you're at war with another sector (before they can enter your territory they need to destroy your space fortress).