I think we can all agree that GalCiv 3 is a solid base. We'll all probably disagree with what we think it's missing or needs help with.
Rising Tide was a fantastic expansion for a sub-par game. It wasn't enough to fix all of Beyond Earth's issues, but it went a long way towards making it fun. First and foremost was its overhaul of diplomacy, something I find is seriously lacking in Gal Civ. The diplomatic currency that's used to build relationships and make agreements is fantastic and fun, but could be improved further, even. Seeing GC adopt a system like this would be fantastic. Not only could you use a diplomacy currency to work with other alien races, but seeing the return of political parties and using that currency to strengthen your party and its political foothold within your empire would be fun. The stronger your party, the better your benefits - benefits you could in part choose with your diplomacy points.
The War Score mechanic - nearly entirely lifted from games like Europa Universalis - would also be a welcome addition. It its current form, Rising Tide's War Score mechanic is fun and organized and frankly, makes sense of war spoils.
I also like that RT brought about new ways to colonize. Imagine a GC race that focused on building starbases as colonies, instead of colonizing planets? That type of "outside the box" gameplay is something I'd welcome with open arms to GC3.
Also RT's affinity system, similar to GC's on paper, is an awesome way to quickly identify a rival faction's path. Seeing a system like this in GC would be great. Your opponent tales the path of Malevolency? You'll start noticing their ships have skulls on them and their designs start looking more menacing. Maybe they even start having a particular visual effect, like flames or something.
What are games YOU think GC3 should take inspiration from?
Endless Space, Master of Orion, Endless Legend, StarDrive 1/2?