For stealing
Gold and
Culture, I
was envisioning that resources are being transferred from one empire to another. Ideally it'd be calibrated so that
(1) each individual spy had a pretty marginal* impact (on the order of -5–10% of one yield in one City each); and
(2) if multiple Civs' spies were spying around in the same city, they'd get in each other way/slow each other down.
I think that if multiple Spies were deployed against you, this would be "annoying" in the same way that early-game Barbarians are annoying: challenging and requiring some work to deal with, but enhancing the overall experience. Certainly the number of spies should be determined in a more interesting/fun manner, and one involving active choice.
With respect to these two points, perhaps if counterespionage occasionally netted you an extra spy (i.e., a turncoat), leaving Spies in your own City would be more fun. I'm imagining an Opportunities-type box: "You have caught the Russian spy Denis! You can return him to his homeland (receive 100
), execute him (0
), or recruit him to your side (pay 200
)."
Obviously, reworking the Spy system would be a large amount of work for Thal (and possibly others). But espionage as it is now is quite lackluster; in fact, I'd put it this way: if G&K didn't have espionage and I had downloaded a mod adding it in, I think after trying it, I'd conclude "Meh. Not that interesting gameplay-wise and doesn't really fit into the rest of the game well." and remove it.
If others agree with me on this, I think it might be worth the time to change.
* – this wouldn't be marginal if lots of that yield were concentrated in that City (e.g., the Capital) – but in that case, you should have a counterspy working there anyway.