As mentioned on tonight’s stream with @yshan, here are my stone-spiral-stairs.zip (21.1 KB). I’ve included a 1, 2 and 3 story version.
It’s built on the idea that each level of stairs happens at a 45-degree angle, and that line could be extrapolated outward for wider stairs. The center will always be static with only the height changing. Width would just increase from the center line outward in all directions.
Hope this is useful.
I just watched the stream and come here to share my opinion on this as I make spiral stairs quite often 
My stairs follow the same approach, but I use differend widths in different locations, below is an example where I coloured the width 1-3 differently:
Procedurally this is a pretty simple aproach and they are defined by width, hight, location and orientation like the classic stairs which are implemented already, aditionally you could use a clockwise, counter clockwise property (I normally use mathematically positive direction ):
for stairwidth 1 to width . start at position central post + stariwidth,0,1 -- assuming 0,0,0 is the block below the central post . for each Step 1 to height . . if step is even rotate 90° . . build straigth slab with length stairwidth + 1 . . move to end of build slab + 0,0,1 -- 1 up
result is a stair with width 2 x width + 1 wich looks square from above:
After 180° its height is above 4 so hearthlings can walk below it:
Optionally you could implement a version without central post:
or even starting with an inner width wich automatically removes the post at inner width = 1 (sorry, no pic saved for that )
Edit: And yes, I just realized, my top row is not built like the formula 
therefore you would replace
. . build straigth slab with length stairwidth + 1
with
. . if step = height and step is odd, build straight slab with length =2, build straight slab with length stairwidth + 1
made a small contribution. I ussally don’t build them but there pretty simple in design so this is what i came up with. I tried to have a maximum amount of turn on it and the villagers are able to use it. It is just 6 blocks high with a 7th block to point out upper floor placement But i don’t think that more then 2 wide really looks nice. i do think that with a wider centre it wil look better with wider stairs.
This is exactly it. Thanks for the contributions, @Thomas and @Doc_Brano! Hopefully this will help lower the LOE for Yang to add it into the game. 
