I quite like that spies have an upgrade, but I really feel like the envoy would benefit from one as well, perhaps coming in the industrial era. It could increase the amount of culture generated from cultural bridges and also be a more effective influencer of city-states. Diplomat seems to be a fitting enough name to me.

I'll pop this in the ideas section later as well, but I thought it'd have a better chance of generating discussion here, and I'm interested in seeing what other people think.

I like it.  Brainstorming possible ideas for a diplomat unit (apart from improved Cultural Bridges and IP influence, as you've said), some other places where a diplomat could be helpful:

- Negotiate Peace by lowering war support between your empire and another empire to which it is deployed.  This would be a mutual war support reduction, not a one-sided "offensive" war support reduction, and which could only be done during peacetime, and would have a maximum amount reduced (10 per diplomat?) or which would slow over time 

- Negotiate Trade by lowering the costs of purchasing resources from one another, or improving the money gained on trade routes between cities (or even granting other bonuses along trade routes like — I can't believe I'm saying this! — causing even more influence to flow along a trade route into a specific territory)

I think "the diplomat" would be a fitting name to the upgrade of the envoy and I think he should provide bonuses to relations when stationed in cities of other nations and provide tangible boosts or options when stationed in an independent people's city (giving a boost to the speed of relations or diplomatic options not normally available?)

Also - somehow I think it would make sense if players couldn't do diplomacy in the early stages of the game without an envoy on the field in opposing territory. But that might be a bridge too far for most players.

Wald

Really nice ideas, not sure how feasible will be but it is noted ^^