Didn't vanilla civ4 have basically no espionage? I haven't played it since warlords came out, and I haven't played that since BTS came out, so my memory is a bit fuzzy, but I seem to remember spies only becoming available at nationhood or so.
Yeah. About the only meaningful thing was sabotage of spaceship parts though, which has always been kinda abusable though and I DON'T want that ever back again.
Overall, I could live with no espionage actually. BtS's system was annoying and it was made worse by the fact that turning it off unbalanced the game anyway (turning too many espy points into culture).
If they brought it back, most actions need to be toned down and I wouldn't want it as a prominent role, it should probably just run off of gold entirely and maybe a building or two, no whole separate economy. It shouldn't be things so trivial/common that it will lead to insane diplo problems (AI in BtS could easily accrue significant modifiers against each other for espionage) and should be present throughout the game.
I really really liked the establishment of "embassies" in civ3 for one, so maybe a minor system with something like that, the ability to steal world maps/investigate cities periodically later on, and maybe a few straightforward missions relating to war intel, or perhaps city state influence and so on could work. For example a way to spend gold on an "espionage" mission that actually decreases a rival's influence with a city state could be all right. Peeking into opponent cities for a gold cost wouldn't be bad either, but they would have to fix things up where, say, all buildings in a city aren't visible to all players anyway. (I'm annoyed if we still have to do this looking on the map, by the way

)
Oh and I actually voted the last option though because at this point I wouldn't trust game developers to get it right. And it's not much of a loss leaving it off all together seeing as I wouldn't want a huge system anyway. It's not just civ, it's again pretty much every game that had an espionage mechanic, just doesn't work too well.