Anything that can be placed on the board, that is not a figurine and does not perform a particular function also required of identical pieces (doors, blocked squares, pit traps). It'd be a rum deal to give someone a single blocked square that is all cool.
Regarding items that fit into apertures, I mean digital creations, printed out and laminated onto heavy-weight stock, that can replace the similar pieces used in furniture. Like these palisades that I made to fit into door bases,

In the case that you want to make something like this, make enough to warrant good use. If someone needs door bases, I can reproduce them very quickly. A good example of something I could do, would be to make a full set of the game's custom doors (iron entrance, wood exit, magical mirrors, portcullises, stone slab) and print them out in high-quality and on photo-paper. I would then attach those to thick mounting board and put that in a door-base. Simply put, you may make digital creations as well as physical ones, just so long as they are all delivered physically.