Ducts, specifically three; one that fits the 40, one that fits the 100, and one that fits the actual duct blocks which are over 100.
The glass textures on all generic shapes.
Moving water to create fake waterfalls.
An entire category of 16-bit textures (wishful I know).
More detailed wood and stonework for decorating such as pillars and trims.
Biggest improvement would be to allow tinting the prop with color sliders, and maybe some way to copy the properties of a generic shape and paste them onto other shapes.
Wallpapers for sure! More carpet type textures for interior floors. Carpets do have repeating patterns anyway, there is an excellent choice of wood flooring already which is great if you want wood flooring.
Maybe different variants of the primary coloured textures with paint degradation, dirty bits etc so they look like they are painted, this would be great for smaller pieces for window frames, making fences etc. as well as for interior walls that are painted. Maybe to look like wood that has been painted with those colours with cracks in the paint and an occasional flake of paint coming off.
And generally less repeating patterns for brick walls etc - on a large wall you can immediately see its the same pattern over and over, but they are great for smaller areas. The bricks seem quite small too.
Def more types of rusted, painted, and corrugated metals. Same goes for concrete and, different types of roofing materials brick, and maybe some sort of see-through grating.
Right now I only use these objects for very peripheral details since both the blockyness of the shapes and the overly clean/easily tile able textures doesn't really work for the more run down aesthetic of the FC assets.
It would be great if we had more variety for the shapes as well. I would really love to have smaller modular arcs so I can more easily include tunnels into levels without having to dedicate so much of the design to them because of the current size of the tunnel meshes we have.