I want half-tile walls. A hallway that is only one square wide is too narrow, but a hallway that is two tiles wide is not a hallway, it's a room. The dimensions in this game are all over the place. I feel like this would help with that.
I want real split levels. I want to be able to build under a platform and place doors and windows between floors. I would love to have a basement that pokes out of the ground a little so I could add windows and window wells. Having to use a slope hack with a thousand bushes to hide the sloped land right up next to the house is annoying.
I want to have a garage without a foundation right next to a house with a foundation. You can do this with platforms, technically, but you can't paint the platform and wall with different swatches.
Other than all that, all I could ask for more are spiral stairs. I feel like spiral stairs are the only possibility, sadly.