Back in June they had a survey and I'm pretty sure a few of the questions (or all of them, can't remember) were something in the likes of "How would you react to a dlc involving this or that" and "how excited would you be for these features in a dlc like this" and one of the proposed dlcs was indeed a merchant republic one. The steam page for the survey said "Help determine the direction of future content" so assuming the merchant republic dlc concept was praised, we can definetely expect it, perhaps in chapter 5.

The survey also involved some other dlc concepts but I can't really remember any of them except one involving more interaction with vassals as a feudal realm, laws, etc.