Espionage

Spies and covert operations could be fun.

Simple example:

Funding is provided by a slider based on how much of Economy that is spent on it (so UI exactly like the social<->war manufacturing spending)

Covert operations could be things like

*Plant spies (same result as a one-sided exploration treaty)

*Theft (transfer money)

*Assasinate scientists (remove some research points)

*Assasinate engineers (remove some manufacturing points on every shipyard)

*Terror attacks (decrease approval, decrease tourism)

*Assasinate foreign dignitaries (decrease diplomacy)

*Search for enemy spies (protects against covert operations)

Personally I never liked the steal technology present in other games, it's too strong and never risky enough, especially when already at war. Trading for techs is overall just better, give something to get something.