Okay, it's hard to choose a top three but I'm going to go with:
1. A Fear emotion, including a system to personalize Sims fears. And because this would warrant a tweak of the emotion system, this would include changing the default state of "Happy" to "Fine" to improve and balance Sim moods to be less elated all the time.
2. A Traits overhaul to make Traits dictate actual behaviors and reactions, not just give mood buffs. They should override generic emotion buffs like decorations and also override generic object attractions. For example, this would mean that a Sim who isn't a Geek wouldn't suddenly start playing video games, and if they aren't materialistic, they won't get a "nicely decorated" buff.
3. Deeper personality system to add likes, dislikes, preferences, and especially individual Flaws. Even if Flaws were done like a second Rewards store and Sims could spend their points there, I'd use them all the time. I don't really use Rewards because the game kind of hands us everything without challenges and consequences anyway so they're really not needed. Flaws however, are desperately needed. And I'd also love a chemistry system too, but I don't know if Sims 4 is capable of that.
Bonus: One more thing that I'd really, really like to be changed, because this isn't something to add but something to actually tune way down or remove....no more drinking glasses of water. It just makes the behavior so much worse and interrupts real gameplay. It's one of the few features that I can say I actually hate. I've actually removed all of the sinks from a house because of this very bad idea. My Vampires are affected by it all the time and it just irritates me endlessly. It only serves Mermaids and not enough for me to care or miss it.
