This is a little far out as ideas go, but I think a muscle / mechanical power system would be a good addition in the game to make T3/T4 industry more engaging, tie it closer to other game systems, and enhance the late-medieval aesthetic of the game. Potentially even an important enough mechanic to centre a DLC around it.
The core of the idea is that some buildings should require a power source - more or less the buildings that currently uses heavy tools.
The first way to provide power to such a building would be to assign a work animal to it: either a cow or a horse. This could work in much the same way as cavalry stables take horses from civilian stables to work. Using a cow would come at a -20% work speed modifier, to encourage the use of more expensive and slower breeding horses. The same system could also be used in crop fields to speed up plowing and planting. Or be a requisite for transport wagons (although that would require a juggling of existing buildings to prevent a hard lock between wagons needing animals, animals needing a trader, the trader needing clay, and clay needing wagons).
The second way to provide power would be to place a building in a windy area. This would be another overlay on the map and would go from 0% to 100% wind - placing a building in a partially windy place and investing a heavy tool would provide wind power but at a work speed penalty corresponding to the lack of wind.
Thirdly, if rivers are added, placing a building next to a river and investing a heavy tool would provide hydraulic power at no speed penalty.
Historically this (loosely) reflects the different sources of pre-industrial power that were used in everything from milling grain to raising trip hammers. I won’t hide the fact that this is partially inspired by Manor Lords, but it’s one of the few parts of the game worth being inspired by IMO. On a purely artistic level, we could have slightly different models for power buildings depending on their source of power: a water wheel if using hydraulic power, a small windmill if wind powered, and a circular mill track if animal powered. Those could be modular annexes to existing models to avoid tripling the amount of assets needed.
On a mechanical level, it provides an additional layer of decision making and forward planning when growing your industrial base to a late-game level, ie: “Do I put my industry close together to make transport easy, or do I prioritise windy/riverine areas?”, “Is the investment in horses and stables worth a 20% speed boost over cows?”, “Is it better to occasionally make heavy tools to power a building, or to continuously feed a cow/horse?” If properly balanced, the answer to these questions would vary with the map and the state of your village, giving the player something to respond and adapt to. At the same time, by using the same mechanic for many different buildings, the player isn’t bombarded with too many confusing details.