(1) Treat stairs as a texture (such as walls, floors, chandeliers) and allow you to cycle between the stair types we have today (narrow stair (small house), dual stair (medium house) or wide stair (large house)).
In this situation SE can add more stair types later if they so please it.
I advise they do the same for the in house pillars. Having more style possibilities can enhance the design.

(2) Open up the balcony door so you can go from inside to outside and the other way around. Also add a trapdoor to the roof.

(3) Allow us to move the location of the workshop and make a separate door for each of the private rooms if you so wish it (great for Fc's that have rooms dedicated to gambling, crafting, etc (just not ERP!) xD

(4) Allow us to make certain architectural decisions like moving the position of the pillars, the windows and even "hide" them if we so choose it! Also, we should have the ability to sacrifice/hide/lock rooms and "get something in return".

I wouldn't mind sacrificing my mansion basement if I knew that doing this gave me 50 additional item slots to be used on the other floors.