While I am enjoying Supernova. There are a few things I miss from previous entries in the GalCiv series.

Spin Control Center

It was a special improvement, I think one per galaxy, that made ships stationed at the planet it was built on appear five times stronger. In my last game of GalCiv II played I got a powerful precursor ship from an event and I stationed it on my planet with the improvement. Every other civilization was terrified of my military might and didn't wage war on me.

Broker peace

It was nice being able to convince other civs to make peace. Sure it was a bit tedious since when one declared war all the others did as well, so I would have to go around bribing everyone to stop killing each other. But it made roleplaying a diplomatic civ better.

Wormholes

I have yet to see one in IV. Not sure how well it would interact with the sector system. But I loved making a fleet composed of a survey ship, colony ship and constructor and sending it into the unknown to settle some new area.

Hypergates and Hyperlanes

They really helped speed up ship travel in your territory.

Crisis events

I felt that these were rather fun to experience.

Missing Civilizations

Sure most of the past ones are here. Notable exceptions are Slyne, Thalan, Snathi, and Korx. The Thalan have a lore reason for not being playable. The Korx as well to a degree. But the Snathi and Slyne do not.

At the very least the Korx should be brough back as citizens of the Krynn Syndicate.

Star Control Civilizations

I know those DLC were polarizing for some. But I enjoyed playing against them and I would love to see the GC IV's artist take on the aliens from Origins.

At least bring back the Mu'Kay, The Measured, Mowlings, and Xraki as citizens.