Impermeable Floor (Update 6): Treated planks instead of metal
In my opinion, treated planks are currently underused as a construction-material, whereas metal is used for nearly everything. I would like to see treated planks used more, especially in cases where it makes sense. I think the new 'impermeable floor' is one thing where it would make a lot of sense, because - at least as far as I understood - you treat the planks to make them more waterproof in the first place.
Maybe you could also add 1 pine resin to the construction cost, as thinks like resin and tar where used historically to get plank constructions like ships watertight - and in my timberborn games I always get more resin then I can currently use anyway :)