

So, why not just make them normal leaders with traits?


In fact, this restriction alone is probably what is hampering Espionage so much as a system in this game, as you can only have ~1-2 spymasters active without crippling your diplomatic weight or relations. It's not the fact that we only get a certain amount of them, but rather we have so many things we are expected to devote them to (Federations, Galactic Community, Imperium support or subversion, First Contact, Espionage, Help or Harm relations for a dozen different empires) while only having 4 or 5 in a best case scenario.
