Awesome that this will be addressed, im sure you have a pretty good idea already of what its going to be like but in my humble opinion I think that what it needs is more flexibility, options to do different things to your opponent,for example

-Sabotage ship production, reset production progress

-Steal tech (randomly)

-Destroy buildings (obviously this would either have to be impossible or require a lot of additional funds for "wonder" building types)

-Incite revolt, perhaps with a time limited but large drop in the influence of the planet

-Perhaps even assassinate leader and blame another player, also it would be fun to start ones own wars this way (slightly wrong, perhaps). A change in character on the diplomatic screen would be refreshing but i guess this would be pretty difficult to implement as they are animated but it would be awesome if next time you spoke to them its the replacement for the guy you assassinated intent on revenge.

-A Blood feud could then develop diplomatically between two races (now i know this is off topic but i just thought of it), in which case it would be a fight to the death.

- deception, maybe when you spy on a race you can give them misinformation such as make it appear as if you are overall weaker or stronger than you really are. Or that there is a fleet of ships heading for their home world when really its heading to a juicy isolated planet. I dont know if the ai is up to this level of complexity though, i dont think it will be able to objectively handle information like this and it will probably just be very exploitable by human players.

Okay those are just a few options. You may notice that these are quite similar to the Alpha Centuari spying options, which is defiantly an example of a more complex espionage system that worked fairly well, but i think Stardock can do better.