I suspect work on GC4 will begin in the next year if not sooner... I know everyone has their wish list... but I just felt I should list out five broad changes that would make GC4 stand out from GC3 for me.
1) Change how the map functions. Frankly, the ol' completely flat and completely open map has gotten stale and I think it creates some technical issues. I think some very interesting hybrid map styles could be designed.
2) Differentiate between races and their functionality more. Have the abilities for races parsed out better and selectable. Having two ability points and then 50 abilities, many of which are extremely weak and some extremely strong has gotten a bit silly.
3) Diplomacy in most 4Xs is a extremely simple and largely risk-free mini-game. Make diplomacy a higher-stakes activity, with the potential for failure. An example could be... that if I ask for a trade deal and I have pretty outrageous terms, I have a low chance of success and TRYING and FAILING causes a penalty to relations for X turns. Succeeding could also be very lucky, and also come with a relations penalty for X turns because they feel cheated.
4) Combat needs a serious overhaul and serious attention this time.
5) Have mod support through Steam. GC3's modding has been a shadow of what it could have and should have been. I also highly recommend you allow people to import ship designs from GC3 to 4, and allow us to save designs as any hull type freely.