As I play my Human Fortress a certain pattern starts to emerge. Oh, it's tolerable when building on the ground floor, but once I start to build up things get complicated. This can apply to any sort of multi-level structure above ground, whether human, dwarven, or even goblin.
Whenever I build a building, I inevitably have to add a roof to it in order to add a bed. Then when I want to add a second story it becomes such a pain when I have to delete every single floor space that I want to make into a wall, slowing down building to a crawl as I tell them to remove the specific pieces of flooring, waiting for them to finish, then ordering them to build walls in their place. This can even cause "cave-ins" (or more accurately, structural collapse) or end up trapping my builders while someone else goes to build the thing they need to get out.
I propose that floors can be converted to walls and vice-versa, allowing me to build a complete roof above, then when I want to add a second story simply convert the flooring into walls. It would also be useful for renovating existing structures. This wouldn't work for floors created by building a wall.
Though a more realistic option would be adding a building that acts more like a roof rather than using flooring. Maybe some kind of thatched roofing?