I have not played this game yet, so keep in mind guys that I didnt check all features so I am maybe missing something.
My suggestion would be if we want this game to be 100% realistic, to use pigeons as messengers like they have been used in mediveal times.
How would this work:
So if there is Player 1 and he writes message to Player 2, message travels there for some time, lets say 4 min per tile.
This could be ordinary message system and lets call it LVL 1 (horse messangers). Chating with neutral lads
And if you have some guy that is friendly Player 3 and Player 1 are good buddies. And they want better communication so Player 3 allowes Player 1 and reversa to build Dovecotes "pigeon house" In his Castle and now their messages travel 1 min per tile, LVL2.
I hope you all got this, my English is not main language so if you have any more question pls feel free to ask.
I think this would make game more unique. We could have small animation when you send message pigeon or horse traveling and vanish after 1st tile