Obviously they'll add in espionage at some point in the future.  So... 

1.  Espionage points that allow you to see what the AI is building in their worlds is mostly pointless.  I don't particularly care about that.  What espionage should be about is this:

AI's diplomatic modifiers towards player

Relative power and production and population graphs

Number of AI colonies, tech level, and number of ships.

In other words, all those things humans currently get for free should instead be something the human has to WORK FOR with espionage.  

This makes much more sense.  In the current state of the game, any espionage system is sort of pointless because I ALREADY KNOW EVERYTHING I NEED TO KNOW just by talking to the AI.  

2.  If you add a sabotage system, the only thing I'd care about is the ability to sabotage AI shipyards.  Sabotaging AI buildings on the planet is sort of pointless, I think, because the AI has too big of a production boost at the higher levels and sabotaging a few buildings here or there won't make a big difference. But sabotaging shipyards... that can make a difference in a war.  

3. Make it harder to trade techs with AI.  BUT MAKE IT POSSIBLE TO STEAL TECHS FROM AI. In other words, allow a system where you can replace research and instead use espionage.  Balancing it will be difficult, but Civ IV did a good job of it to the point where research-through-espionage is a valid way of beating the game.  

4.  The ability to place spies on enemy fleets to keep track of their locations.

5.  Make it more difficult to explore AI space with conventional means.  Consequently, espionage becomes much more important.