So according to what has been revealed so far, diplomacy is getting a good update. But what about espionage?
GC2's espionage system was more like an afterthought, it didn't really add anything to the game except allowing the AI to annoy the hell out of you.
If the espionage system is going to be revamped I urge/plead/beg/recommend you to have a look at how it's done in Imperium Galactica II because it's espionage system was actually worth using.
Edit; ground-combat was better in IG2 too. (Not talking about the real-time aspect though, I know GC2/3 is turn-based.)