I'd love to see more complex gameplay. I miss the grocery system. I miss having to work to raise relationships and skills. I have so many mods to increase the complexity. In my games my sim's income is lowered, the cost of university is increased, pregnancy odds are manipulated, there is a risk of accidental pregnancy.
I want the chance of things going sideways. I want sim's traits to change how my sim acts, not just what interactions are available to them. I want some personality, some attraction, some challenge, the option to have random events. I want the silly jokes, the slightly off center jokes. I've been playing The Sims since it came with our first computer when I was a kid and I'll be honest, I 100% forgot about the person in the cake thing and now I feel cheated that it's not in TS4.
