I've often wanted to communicate with people I'm buying/selling food to/from, but since they're usually in a different realm I don't have the ability to send them a letter. I'd like to be able to attach a letter along with the food/gold that's only delivered when the caravan arrives. Possibly without the ability to reply and just to be used as flavour text. Something like:
"Fine Fatmilakian fish and berries, preserved with salt from the Panabuk mountains. Your next shipment shall arrive in two weeks time at the same price."
Nice idea, but should wait until estates are done.
The ability to contact the lord on the other end of the caravan route is much needed. There have been some efforts to work out a system that would allow you to do this, including a way to send a "trade offer" type of message instead of blindly sending out caravans.
There are guilds and religions to allow you to contact other nobles, and you can always talk to them through their respective government officers. This is restrictive, but I thought that was intended.
I don't think that the new caravan system is revolutionary enough that it should allow Lords to contact all other nearby Lords without proper introduction, such as visiting their regions.