Changes that would be great:
The ability to toggle whether I want my walls/floors to go through the ground or not. It's very annoying to not be able to place a wall just because the house is too close to a small rock, pole, etc.
Make fences have the ability to connect to each other, and change the fence length to the size of half a grid. It's very annoying trying to make fences connect to each other perfectly, and it's even more annoying when the fence is too long or short because the thing I'm trying to connect it to isn't the perfect distance from the fence.
Similar problem: make the kitchen cabinets connect to each other automatically.
Similar problem: make the gates and the bigger walls connect to each other automatically.
Additions that would be great:
Roof blocks/slanted blocks. I don't want every one of my houses to have a square, uninteresting roof.
Paintings. Probably shouldn't make it customizable because people would put very lewd things on it.
Showers, to go with the tubs.
Being able to place stuff on some of the shelves; doesn't work sometimes.
A new type of wall that doesn't have those stripes at the bottom
Keyboards to go with the computer monitors
Ability to color walls with a paintbrush tool (maybe store it in a future weapons inventory?)
Ability to place the stairs and doors where-ever you want, instead of only in a wall or floor
Ability to select spawn-point
Ability to toggle chat filters
Carpets that can be placed on top of grid floor blocks, and doesn't appear on the ceiling below.
Better jump physics; it's very weird to jump on top of a platform but slide right off
Better car physics; feels really hard to drive
Working lights (even though there's virtually no lighting system in the game)
A completely flat area
More furniture, obviously
Ability to open closets in the game
Make that white trashcan bigger, like who has a trashcan that small?
Add smaller ramps to ramp list
Road blocks (not roadblocks, placeable roads)
Please, please, PLEASE add a PC edition to the game.
Better character models, for example the girl's head twists really weirdly when looking to the side