I'd just be happy with some sort of draw-able stair case to be honest. I'm not sure how you'd navigate or work that out in game but it would be an interesting feature and would open up design so much more. It's hard being restricted to a 1 by 4 straight line or 2 by 4 if you have two stair cases next to each other. That or what everyone else is suggesting. Split levels would be awesome personally, because my own house has split level stairs and I'd love to be able to actually build it on the sims 4.