- Open world
- Story progression. Inheritance system so that wealth is not destroyed when a household with relatives wither. Dynasties that make a decent attempt to survive and pass on their wealth, both material and cultural (cultivated wealthy families should have the ability and the will to provide a better than average education to their progeniture, and have it more skilled as a consequence).
- Bring back TS2 personalities. Traits are really good, being able to talk about what happened to them recently as well, and they could remain. But TS2 personnalities really don't cost much to implement and really made sims very unique; they were able to chat about more than there job, their 2,3 interest and their short term memories, and have more than on and off interest, their chit chat were usually deeper than TS3 "deep" conversation about favorite colors. Emotions are nice and all, but if Sims have a personality so shallow it cna be summed up in 3 traits, it's just like bad acting from empty shells. Traits allow to perform a few gimmicky actions. TS2 personallities potentially add a tweak to each everyday live action
- Better relationships. TS has it right that relationship are not linear, and romance, though not completely orthogonal to friendship, is an other dimension. But it could go further. You could have social kinship (a hobo will have a hard time being invited by rich people, let alone marry one; at least among those who follow the norm). You could have physical attraction, which instead of being a static and universal compound of fitness, alchemy and perks, could evolve when sims interacting with each other or witnessing each other dancing, acting, practicing sports (all with modifiers according to their personalities)
- Better autonomy. A Sim left on their own with every thing they need available should be able to consistently go to the bathroom and survive hunger and embarassement. Would also help implementing better relationships and personalities without overwhelming the user with menues.
- Memories. Any system is great, just keep it up. Also, more memory interaction, reminisce could be an actual memory rather than just a generic action you do with old friend; bring back the diary (having every sim wanting to vlog is ok, having every sim keeping a pen a paper diary probably isn't though, so I guess you'd need good personalities to make it believably uncommon)
- meaningful conversations. be able to ask Sims more than their job, their partner and their astrological sign. That three are great, but not sufficient. Ask about children (and learn a thing or two about them each time you do), talk about friends, have "get to know" interaction that have you actually get to know the other Sims (requires something to know in the first plaace, so again, better personalities).