1) Elevating the base of platforms. If high enough, sims should be able to walk below them.
2) Being able to place doors/windows between floors.
3) Being able to connect rooms/walls of different levels.
Not likely to be implemented in S4, but wanted nonetheless:
Reworked grid. It would be nice to be able to build walls that are 1/2 or 1/4 the length of the current grid. I often end up in frustration where +1 is too much and 1/2 or 1/4 would've been perfect.
Just like wall height, options in wall thickness would be neat. Having thicker outer walls would look a lot more realistic, and thicker walls would add enough space for window niches/sills with slots for things like plants or lamps.