I can't count the amount of times when I had to create ridiculous monstrosities in order to get to a second floor of anything I was building. Currently we only have straight staircases, which do fine in some cases (and wooden Leyawiin stairway has been a great addition to housing, btw), but they all have a significant disadvantage - they take up A LOT of space, since you need to fit the staircase itself and leave some free space on both ends of it. It can be solved to some extent by building improvised structures to walk on from pieces that weren't designed to be used as stairs, but it takes up dozens of furnishing slots and mostly looks hideous.
Spiral staircases would solve a lot of problems and allow for creative and elegant construction, since they take up half the space of normal staircases (or even less) and sometimes just look amazing. They're a must-have for any tower or castle-like structure and even for some small homes.
P.S.
Also the fact that there's exactly one kind of roof piece available is kinda frustrating.