So, here's something to consider with how diplomacy currently works, and I assume it'd be an easy fix from a technical standpoint...
Diplomacy between warring parties? It seems like it should be a little more limited, a little more tense, no? Buuuuut as you might know it's actually almost identical to diplomacy with friendly civilizations, except that you're shooting at each other between trading sessions.
Enemy civs will continue with trading techs, ships, and so forth as if nothing has happened... But, perhaps the most egregious aspect of this is that you can buy off invading fleets in mid-flight. I was just running a playthrough as the Yor and my forces were a wee bit overtaxed by all of the negative attention that my Diplomacy (un)skill had earned me, and right in the middle of a Terran invasion I had a cyborg-esque
brainstorm--and I bought one of the invading Terran fleets and immediately turned it against the one right next to it.
Now, perhaps I could see some future Espionage mechanic that allowed us to bribe enemy units, but this was a blatant, public transaction that I'm pretty sure the Terrans would never have agreed to, had they been in their right AI minds. The worst part about it was that I purchased units comparable to my own at slightly less than 10% of the normal rush price at one of my own starbases. This... has the potential for rampant abuse.
Moreover, I'd recommend that the "fog of war" be extended to unit listings--possibly planets too--between non-allied parties, except as potential offerings in a peace settlement. As it stands, it's very easy to run through an enemy's diplo window and get a full readout of his assets and capabilities without ever having to scout much in-game. This might be fine between allied civs, but it's a real no-no between people who are trying to kill each other; it would be even worse in multiplayer. I've seen a couple of posts talking about how strangely the AI weighs planets and the like in trade deals; this might be a simple solution to that problem as well, by making them off-limits under most circumstances.
There's my two cents on that... I'm running as a Spelling Nazi on my next playthrough and compiling a list of typos, but that's another story for another day.