This idea would be great, some sort of modularity to it would be nice. For example, if the upgrade slots a building has are based on the size:
Small Buildings - 1 Upgrade/Specialisation Slot
Medium Buildings - 2 Upgrade/Specialisation Slots
Large Buildings - 4 Upgrade/Specialisation Slots
Maybe the upgrades can be like capacity/balcony/defensive tower/iron alarm bell or various buffing aspects, but by restricting how many a building can have, but also allowing variation, people can have similar looking towns but buildings specced to different playstyles.
Eeeehehehrhergerhe I'm getting all excited just thinking about how much this game can advance with community suggestions now, this is awesome