A stone building tier between Planks and Metal would be a great addition. The durability jump is currently too large: Plank foundations have 11,000 HP, while Metal foundations jump to 36,000 HP.
Stone would fit perfectly as a middle tier. For example, stone foundations could have around 22,000 HP, while stone walls could have about 7,200 HP, placing them nicely between Planks and Metal.
To match this new tier, a stone pickaxe could also be added.