The customization options for objects in The Sims 2, where you could mix existing textures, patterns, and colors. Obviously, The Sims 3 system was much better, but I understand that it might have used too many resources. I think expecting a system like The Sims 2's is not unreasonable, and in fact, I came to believe that with Dream Home Decorator, we would finally have it. However, if it wasn't implemented then, it's unlikely to be implemented now.
Cars (and obviously parking spaces and garages). I don't even expect cars that you can drive like in The Sims 3. Just an object that you can have stored in your house, and it appears with you at the places you visit. This feature might seem quite basic, but it has been ignored for so long that I doubt it will ever come.
Differentiated height between teenagers, adults, and seniors. I also had a small hope that this would be implemented with one of the expansions focused on life stages; unfortunately, it never came.
I think those are the three main ones. I'm somewhat disappointed because I don't think they were too unrealistic and added a lot to the game, but I'm almost sure they won't happen. But I could be wrong; less than a month ago, there was a fourth one, which was creating multifamily lots. And similarly, there were already expansions where their inclusion seemed ideal, so I thought there was no hope. Still, it's finally a reality, so who knows.
