One thing that always irked me about spies (and to a lesser degree assasins) was that a spy of a given faction could spy on other factions with the same degree of succes regardless of the culture of the faction being spied on. Realisticly it would be drasticly more difficult for a Roman spy to infiltrate a Carthaginian or even worse a barbarian settlement and build up a network of reliable informants.
So maybe agents and particulary spies and assasins should have a trait "knowledgable about [insert faction]", reflecting that they would best be used against factions that they're accustomed to dealing with. Aditionally agents should have a bonus against factions that are part of the same culture.
For diplomats, there is already a translator anciliary, but the bonus it provides is not faction (or culture) specific. It gives just as much a bonus for Greek vs Macedon wich doesn't make sense.
Is this possible?