There are a few things I would like to suggest. Not sure if anyone else has suggested them, but here goes.

Drawbridges
3 types of vine wall options in the menu (One that is solid covered vines, one that is right diagonal covered and one that is left diagonal covered)
Suspension bridge large enough for a large dino to cross (hanging bridge but 2 floors wide instead of one and higher entrance points)
half length stairs, walls and ramps
Stained glass windows with customizable colors (like stained basic design, but with three color choice depending on the color of dye you use when making it)
Keep greenhouse effect windows to build a greenhouse with all the options of the keep items (such as curved wals, ceilings, doors and floors, regular walls, ceilings, floors and doors but all made of glass with greenhouse effects)
Inverted round rooftops
Small, medium and large curved foundation pieces to make different sized diameter circular towers (of course wall, ceiling and battlement pieces to fit them)

Edit:
Also the banners should be customizable with your own art instead of regions
Windmill blades that can be painted or dyed would be great too.
Ability to paint roofing or color roofing with different coloring. (I want purple roofs for my castle towers please ;))