I like the current spy, envoy, governor, alliance and diplomatic systems. Sure, they range from 'great' to 'needs some work', but they're all very solid. And man, do I not want more bloat. Not at all.
But I wish some of these systems integrated with each other a bit better. The current game feels a bit like first or second edition D&D at the moment, with lots of perfectly good sub-systems, but that all work completely differently.
That said, there is better integration now. Spies can reduce envoys and hinder governors; spies, envoys and governors both come (mostly) from civics; there are ways to get extra spies, envoys and governors outside of civics (if you want to lean into those features); and the second tier government buildings really pull things together. But some more integration would be better, so there are less different subsystems, and more ability to trade off one thing for another. I realise it's very subjective what's right though.
If I could have one thing, it would be the ability to assign envoys to Civs as well as CSs; and be able to reassign envoys periodically (e.g. When you change governments). (Or is that two things?)
On spies, man, they really are just so powerful. Optional, expensive, but powerful. I also like the simplicity of how they work. My only gripe is the strategy around them is a little limited. The only real decision points I see are: do I build them? Which Civ to I place them with (which basically boils down to gold versus diplomatic combat bonus versus stopping someone's victory condition)?
There's no strategy with promotions, because they are random and the 'best' ones are known. There's no real decision between attacking or counter spying, usually you only counterspy to level up or if you have a spy with quartermaster promotion (or maybe have Zimbabwe or a space port). There's no strategy about which specific city you place them in, either you pick the obvious city (spaceport) or all the cities are the same ("bother, they all have spaceports").
What would improve spies IMO is this;
- maybe three spy units, one general one like we have now, one "saboteur" focused on just rocketry and IZ disruption, and a quartermaster that boosts other spies and is better at counter spying;
- each type of spy uses up one of your spy slots.
- for the general spy, a normal promotion tree, and you choose the promotions;
- you need certain promotions to run certain specialised missions (e.g. steal great works);
- Spies give more visibility if they're in a capital, and have more chance of success if you have more spies in the target civ or in the target's allies (although I guess listening post sort of covers that already).