This is a design concept I was kicking around while playing civ5.

One of the things that you can do in civ5: if you know one country is sending an army to go sneak attack another country, you can warn the target country so they can plan appropriately.   While that's a nifty concept, there is no way that the AI is sending that message about me (at least not accurately) because they don't have the advantage of being able to just look at my current scripted intent, and rat me out.   

So all that said, that brings me to my most recent brain child.   What if you could "mark targets"?  For example, you could pick a city that is your strategic desire to acquire.  (the AI could do this too).   Or you could mark an army that it is your strategic desire to eliminate.   A quest to accomplish, etc.   This could give you bonuses that accumulate over time when you attempt to achieve that objective.   Perhaps something like a 5% attack bonus per turn after marking an objective to your attack and defense when interacting with that objective, cheaper tactical spells, or whatever else you want to implement.  

Now,  for a game like this one, you wouldn't want to use espionage because that's an evil word 'round these parts.  We could make it cooler! Global enchantments that offer ways to clue in to these tags like the following: (just a few examples)

"Clairvoyant tendencies" : Gives you a 5% per turn chance of gaining insight into the current intentions of an enemy civilization. 20 mana, 1 mana per turn.

"Omnipresence" : Grants you great knowledge of the pending future.  You know all the intents of the other civilizations.  700 mana, 20 mana per turn.

"Tower of Iron Will" : Grants you a 95% resistance to spells that would reveal your intentions to other civilizations. 40 mana, 3 mana per turn.

"Alter Fate" : Sets the intent of an enemy civilization to the target of your choice until dispelled. 300 mana cost, 15 mana per turn.

Many of these would be very very well served with global spells and a Global spell UI, like so many other really cool spells that would be fun. (see my post about global enchantments) 

I Think powerful spells like the proposed "Alter Fate" and "Omnipresence" would be better if Strategic cast times were implemented.   I believe it would add a feeling of accomplishment for the really big stuff that we don't really get from anything but levels at the moment.  That's just my opinion though.