Earthen building materials! Stuff like Thatch, Cob, Wattle and Daub, Mud Bricks, etc.
It would also be nice to have much more realistic block types for building with, like wood/steel beams that can be used to make the framework of a structure, then wall and roof blocks that surround or fill those. Think how houses use several wooden beams to make a framework and then cover those either side with different internal and external materials, such as brick externally and drywall internally. Also, how medieval buildings used wooden beams with wattle & daub in between to fill the walls.
The early-game is so lacking in terms of construction materials and techniques so these would be awesome to help spice it up and make towns look and feel much nicer.