I think there needs to be new buildings as well as ships. And planetary upgrades.
After having played Battlefleet Gothic, the spinoff game of Warhammer 40,000, and looking at some of their other projects from Forge World, it comes to my attention that planets, being as large as they are, ought to have defenses of their own. Granted, nothing too ridiculous but it wouldn't be unheard of if they had something.
So, for another planetary upgrade, something along the line of surface-to-orbit defense systems. It doesn't have to be specifically stated and could change or exist depending on faction (since Vasari rule from orbit, they could have something else instead; more on that later). I imagine the tec would have missile silos and some kind of magnetic accelerators/mass drivers to launch objects into space and to attack nearby ships. Obviously it'd have a limited range. While the advent would have likely beam weapons and psychic powers.
Of course, as a planet is bombarded from orbit, I imagine these defenses would diminish in effectiveness.
I know we've all seen the orbital elevators which make for pretty but otherwise meaningless additions on the planets. They are neat to look at. However, after playing Conquest: The Frontier Wars, it comes to my attention we don't have orbital platforms of any sort. Wouldn't it be neat, I feel, to have a third pool of buildings (right now we have defense and then structures)? Orbital platforms that could render a variety of additional benefits. Perhaps defense platforms (whicht eh Vasari would definitely use) like flak guns and CIWS for fighters and missiles or autocannons for bigger things that get too close. Maybe supplemental research stations. If for balance's sake, they wouldn't open up new slots but instead would speed up research. Can moderately increase population, et cetera. Additional or supplemental repair facilities. There are a lot of possibilities.
Then I think we ought to consider the possibility of satellites, deep space stations, and other things we can put outside of planets or beyond a planet's gravity well. Or where there are no planets. Buildings that could be specific to, say, certain areas like in an asteroid field you could put disguised buildings or hidden buildings such as listening posts, fighter bays, or research facilities. Defense satellites that are deployed like a starbase as something mobile but when deployed become stationary regardless of where. Laser satellites, torpedo satellites, missile satellites, et cetera. Spy satellites, even.
I also think we need ANOTHER class of ships, one that is between cruiser and capital ship. Something like a smaller capital ship that doesn't level or have nearly the same fire power, but has more than one weapon system aboard and greater survivability. And like two or three kinds per faction. Like one ship would have beam weapons and missiles. Another would have flak guns and fighters. They would also be built from the heavy ship yard, in my opinion, so greater use can be squeezed out of those things.