Although I doubt it will ever happen because it would just be way too far down the list of priorities at this point, I would like to see granite and fieldstone walls of various heights (both "dry laid" so no mortar shows and with mortar showing and without and without moss growing on them) and in a couple of different colors of stone. This would perhaps make it easier to build houses that look like they are built into the side of a hill or on top of a hill. It would also be nice to have a much greater variety of pillars and arches to be able to build cloister-like sections on a plot. It would also be nice to be able to build with floor tiles that could give you the look of a pool set into the floor or into bare ground or grass, obviously including tiles that would be part water and part stone or tile, part water and part grass, or part water and part dirt.
It would also be wonderful if the existing sets of housing tiles could be completed to include missing tiles.