How are mods going to be distributed and made available to users? It's come up before, but there hasn't been a real answer, and there's nothing about it in the design document at all (far as I can tell, it is pretty gigantic now).
If mods are going to be a more prominent "back of the box" type of feature, so to speak, how they're going to be distributed and consumed is important. Right now I have the distinct impression that "go to the mods forum, find the thread for the mod you want, download a zip file from that, and extract it to some folder whose location varies based on where you bought the game" is on the table as an answer, and that's just not good for 2016.
Especially when Steam Workshop is right over there, offering prime discoverability, easy installation, automatic updates, and feedback tools for mod authors.