What’s probably hindering their ability to add different stair types is the different wall heights. The straight stairs account for taller heights by adding 3-4 steps as you go up, but for spirals and 90° stairs that wouldn’t work in the same way. In my opinion, 90° stairs are something we will see before spiral staircases. The game already attatches a landing to the top of staircases that do not have a room directly attached to the top step, I figure that could be reworked to create a turning point at different levels of the staircase(lower, middle, and upper maybe?). I’m sure it would need a lot of work to be compatible with all 3 wall heights too, but I think it would require less than spiral staircases.