What else would you want to change / can you think of that would to cool to have regarding espionage ?
I definitely think couple tweaks could be implemented about Espionage without overhaul of entire system. Automatic TRADE EMBARGO is a solution ...
#1 When you catch a players spy's destroying your provinces that it should be automatic TRADE EMBARGO against whom ever deployed the spy.
#2 Gold spy use against your country is NOT posted in the Newspaper > there should be some indicator such as an automatic trade embargo when this happens, this would also be a nice indicator to aware players of whom is using gold against them when they see an airfield/building destroyed and an unexpected trade embargo with a country.