1. Fix Diplomacy
Remove Diplomacy Lockout
Add Diplomacy Fatigue and Scale Number of Interactions with Potential value on trades. If I have no interactions with the AI over the last 20 turns I should get the full value of a trade. If I have 1 interaction I should get the full value if they got the better deal last time. If they got a worse deal, I should get a 10% reduction in what they will offer me for something or have to pay 10% more for something I want from them, more recent interactions = higher penalties & lower values. The whole 20 turn idea effect cooling period is good but the lockout is a copout.
Also modify this with your relationship slider, if you have great relations then you pay less of a penalty, if you have poor relations expect to need your checkbook as well as your wallet if you want to deal.
You should want to hear from your "friends" and not want to hear from enemies. So if you have an alliance or are in the highest relationship zones, then the above fatigue should not apply. Like a friend you keep bugging to hang out who has other stuff to do. You should be told "That is all for now" after a series of successive recent interactions. Not one interaction.
Races that are more diplomatic should also have a modifier here too. Races that don't like to talk should charge you more for the privilege but then also shut the hell up too.
As has been listed, Diplomacy should not be how you do espionage and learn about another race and all of their assets. You should only know what you've seen. However, if you want to know they should be able to price that information like how many planets do you have, or how many ships do you have? Instead of begging for money every 3 turns the AI should have lots of diplomatic products to sell you. Want information on how many planets another race has? Well just ask your neighbor, maybe if you are allied with them they'll sell this to you cheap.
You should be able to threaten and coerce and negotiate. If an enemy wants to go to war with you, they should name a price which would prevent it, or be able to do the same to you. Commodity resources should also be tradable for fewer than 50 turns or longer than 50. More options is always a good thing.
2. Fix AI, Each race needs a few different AI engines, one for how to spend their money, one for how to build their planets, one for how to build their fleets, one for where to stage their fleets and one for getting "friends" The AI who builds planets needs the most work, for if they can't do this on an individual basis effectively they will never compete anywhere else. I think this should be accomplished by decision trees, but I could be all wrong on that. The first 100 structures any AI builds should include probabilities to build certain structures to get them off the ground and then it should be filling gaps as efficiently as they can based on the tiles of planets. Certain planets with really good size or fit for a certain kind of specialization should begin to be specialized by the middle "age" What are the choices? Right now the decision for each planet should be production or research. Each planet should have a minimum of 2 factories but then be designated as research or production. Special planets which should be designated by the AI should be built around the specialization like trade capital or research capital or capital whatever, but this should be after the age of expansion and the AI already has a production foothold.
Fleet AI, Early on smaller, faster, more ships, these should be shooting down other empires colonizers. If there is a choice between attacking a human colonizer and AI colonizer the AI should prioritize slapping down the human colonizer. If you attack a colonizer inside your SOI it should be legal for all parties, AI and human, while it should also be just another "-" on the diplomacy modifier it should not immediately lead to open war, or cessation of diplomatic relations. If you do not have open borders you should expect the AI to target your colonizers for destruction outside your SOI. Simple as that. This would also slow down the stupid-fast expansion and all of the other game breakers that come from that.
I might be wrong but it feels like the AI has no concept of borders or can even see these. Either remove influence border display and relegate this to a display on the planet itself or fix it so the borders mean something with benefits and consequences. Colonizing a planet inside of another empire's SOI should carry a diplomatic penalty and it does not appear that it does.
Later on, fleets need to stack and play more intelligent. Build manager also needs to take meta data from the cloud or popular downloads from the nexus or whatever to help it determine what to build and it should be able to change builds on the fly. 64-bit right? So the AI never truly needs to build the same ship twice, why not let it build dozens of variants. Oh yeah RAM is a thing, well figure it out.
3. UI so why can't we have the bubble circle slider right on the Main UI when we click on a planet? Why do we have to go to the govern tab? Save the Govern tab for fine tune and detail data, we should be able to get a quick summary data and adjust individually right on the main UI.
Research UI, stop making me click Research and then Choose New Tech, If I am in the Research screen I'm already there to do that it is redundant.
UI size, yeah make it bigger.
4. Ship battles, Would like the ability to control targeting in real time so as to maximize an advantage. Yes the AI will never compete but it makes watching the battles more fun, and would not needlessly drag out the battles. I'm not suggesting a whole new level of depth just a mini tweak to override the already established targets with new targets on the fly. Very low bandwidth decisions would not affect multiplayer much either.
5. Earlier invasions. I don't even know how deep you have to go to get invasions but in my 100+ turns I've always been expanding faster than I could turn around and just invade. Invasion should be easier than expansion and cheaper too, which is why defending your planets should be a thing and isn't really at this point.
6. Starbase spam/constructor spam yada yada
7. Planets as logistics points. NO. We don't build ships on planets anymore, We don't dock them on planets anymore, NO more logistics, yes they are big globes in space but still, you get fuel, weapons, repairs, personnel from starbases and shipyards not planets. This would slow expansion even a bit more, and it would make more sense too, also it would add tactical necessity to these structures and keeping them alive.