As you start planning your next iteration I would like to suggest complicating the planet types by adding different atmospheres and mantels along with planet class. With the intention of alien civilizations being suitable for one type and not for others, terraforming aside, the goal would be the ability to live amicably in a given solar system with an alien race who posses no threat of colonization of your necessary planet biome. Such situations could make for natural alliances and neutralities giving depth and immersion to the game and expanded victory conditions other than the conquest of everything. While evil races may be able to use every biome, not by establishing colonies there but subjugating into slavery their defeated foes. You could actually liberate the Torians.
Additionally, solar systems should resemble a system model with the planets actually moving, revolving around their star. Some even retrograde. Doesn't matter that in real life it can take days to years for some planets, its a game and a good addition. I like how you made the black holes in GalCiv3 now just expand that to solar systems. On the campaign map you see the rotation of all the planets of a system revolving around its star and the graphics for which races occupy the system, click on the system and it brings you to that system's page where you see more details, ships and starbases and such but it is not rotating. Each system then could be defended in a different or unique manner
Incorporate two movement types, interstellar and intrastellar. Example, Inside a solar system ships are not flying about at warp speed but using thrusters.
Thank you for your consideration