I've made a few maps now and find it a fun and challenging experience. I do have a few suggestions. However, these are all cosmetic. More carpet tiles of various colors. Maybe green, purple and/or blue. More chairs. More windows, preferably as decals so they can be used on any wall. Especially gothic arched windows. NPC decal/doodads that you can put behind counters or use as impassable obstacles. Maybe horses, too. That's all I can think of at the moment.
More aesthetic options can only be a good thing. I wonder if the developers would be open to including player-made decals into the game...
I can see why they don't allow player made content because copyright issues and nsfw content would be a nightmare. Probably best for us to suggest items here. However, player customization as far as tinting and coloring items would go a long way to expanding the current toolset.
Oh, I actually just meant adding player-made decal packs into the board editor, not allowing players to insert whatever they want into a board (since that would probably require changing how the game works, although it would be an even better way to do this).