I also want to put in a plug for getting this into single player. Sure some of these are features that would require online like a mail system and areas while currency and NPC types have nothing to do with online - it is just adding/changing game objects.
I wanna dwarven blacksmith in my town that I build that gives me a discount on using their shop. More immersive than plopping all the stations down in one room like I normally do.
i can look into something for single player in the future. you can always host a server locally to run the plugins. i have been working on an npc helper system as well. will allow you to give resources and Coin to npcs to auto craft things for you. this plugins is still a long way from release tho.