Two-Type Walls/Flooring

It could be a nice feature to integrate two-type. For example, when you place one type of wall, Wood, on a wall of Stone, you'll have the front facing part of the wall of that type, sharing the strength of the highest value (the stone) and allowing you to have interiors of wood with exteriors of stone.

It might be a little complex at first glance, but it could provide an extremely dynamic environment while offering more architectural choices, though mostly aesthetic.