If I recall correctly, it was during the discussion with the CAI team.
When I refer to my notes, in the chapter about the Campaign, I can only read :
When the subject has been discussed, the answer was clearly no from the CAI team.
Adding a "trading region" feature need to modify the CAI to make it able to use this features.
Modders cannot do it because they have no access to CAI.
So, only CA could do it.
About other diplomatic options, I cannot recall (Modding Summit was more than one month and a half ago).
But, each time that it involves deep code modifications, they are quite reluctant.
So, if players really want to see such feature, and they are many, maybe CA will do something, like they have done for seasons.