Water management is probably the farthest frontier to conquer !
Fishing ponds with flood gates where widespread especially in small ponds. You woul grow the fish in the pond, drain it partially and pick up the fish in baskets.
irrigation : could add some flavor to farming (granted this would almost involve some kind of layers and bring the game into another dimension) but worht a thought though.
Water moats : you can build nice walls dipping into water but on flat lands how fun it would be to create moats at the foot of your walls and forts in the low lands.
Rivers : would mean fords, bridges, ferrries. Watermills, flour mills and forge jackhammers…