Villager building/gathering...

I think creating a blueprint mode, which would function like creative but instead place phantom blocks/gathering areas, and allowing villagers to assist you would be phenomenal. The caveat being you could only have 3-9 villagers at a time within your chunk working only while you are present, or whatever is necessary to keep system requirements low for the many users who may not wish to invest in high end equipment. Creating an AI editor that designates what chests a villager can gather from, as well as a Pathfinding editor (to eliminate the need for Pathfinding coding) could simplify the process. In essence you lay out the path through a specialized block/blueprint mode and the villager uses it to go back and forth between the worksite and chest with building materials. The player should have to produce all items necessary, as well as build scaffolding for the villager to utilize. I think it would be fairly simple and should come in an update that you charge for in order to create a sizeable boost to development while also creating an entirely new game in the process.