What are the differences between messengers, diplomats, and ambassadors? How much influence is conferred by each visit to a city state? Are there diminishing returns?
Different technologies unlock different units. Messengers are unlocked with writing and have a movement speed of 3 (ignoring terrain movement penalties). Diplomats are unlocked with the printing press and have a movement speed of 5 (ignoring terrain) and Ambassadors (unlocked via Telegraphy) a movement speed of 7.
They all grant 50 influence when you conduct a 'trade mission' (using the same game mechanic as great merchants, minus the gold gained), the difference being their speed of movement.
This decision was twofold: 1.) I don't want players to avoid technologies because they can spam older (and cheaper) units like the messenger late game and 2.) higher movement speeds roughly represents the increasing speed of communication in the post-printing press world. If 7 seems like a lot of movement, it is but it makes communication seem fast!
All units are 'captureable' by barbarians and rival players and remain as Messengers/Diplomats/Ambassadors (i.e. you can capture them back).
Each new unit replaces its obsolete predecessor (i.e. you go from Messengers-->Diplomats-->Ambassadors during the game).
All units and buildings have 'unique' models (mashups of other units, like settlers), artwork and icons.
Could they be used to trigger city state quests?
Could we devise quests that only they can complete?
Could they could be sent to rival civilizations, improving relations by, say, 1 point?
These are things which I hope (eventually) to add in once I learn how to manipulate the AI, especially the first one. I'm not sure how to do the last one because I'm not sure how to code it in (as there is no 'gift unit' option for rival civs, only city-states).