I suggest the following change in villager mason:
In his exchange tab, place asking for stones and other construction materials for the player (300 stones or 300 wood) and he will pay few emeralds (5). The difference would be, that this villager would be programmed for him to personally renovate and increase the buildings in his village, using the material purchased from the player in real time, in the same way that a player would build. With each exchange made, it would allow an upgrade in the village, with a limit on how many times he would make this service available until the village is very different from how the player found it. I saw a mod that does something similar and would find it amazing if it had game.
A mod being able to do something doesn't mean an idea is good for the standard game. This is a game about the player building, not hiring people to do it for him. That's what multiplayer is for. I also can't imagine the amount of programming needed and incessant time-consuming testing just for this one feature. I know the "it would be hard to code" argument is not that good, but the effort really isn't worth it for something the player should be doing.