One idea I had was that you could select other players' units and bribe them to become your spy for a certain amount of time. This would be subject to increasing costs and lower likelihood of success for more powerful military units. All non-mechanical units could be bribed. After bribing the unit, the original owner and the bribing player would have shared control of it, although this wouldn't be visible to the original owner. Both players could give orders to the unit, but the unit couldn't attack either player. If a spy was garrisoned in a building, the production and research queue is visible. Unfortunately, shared control isn't implemented, but if it is, this could be a good use for it.