What would you like to see that most likely will not happen

More than anything I'd want traits and likes/dislikes to actually affect sim behaviour. It should be more than just a positive or negative moodlet depending on what they do.

Especially since they can only have three traits instead of five, it should do a lot more than in sims 3, not less.

Would be great if lazy sims could actually do any kind of chore faster but at worse quality, and take longer on the stuff they like. Could enjoy just lazing around in bed when they wake up, not getting out on their own unless they have work in an hour or have a need to tend to.
Should be able to lay on the couch while watching tv and do other stuff in a more lazy way. Being lazy could be a whole lifestyle.

A hot headed sim should enjoy fighting games or other stuff to blow off steam, they could be very sore losers in any game they play or any competition. They should not be so fearful, anger should be their first reaction instead. Angry because its raining, angry because they see a ghost, etc. They should be the kind who'd try to punch a ghost and it goes right through them, sims who act rashly in anger.

Evil sims should actually be able to do evil things, maybe have unique opportunities in any job that only an evil sim would think of.
Maybe be able to use some inventions for evil purposes.

Heck, I think traits should unlock unique inventions or other crafts that only someone with that trait can think of. Like only an evil Sim can build a doomsday device, only a lazy sim can invent a chair with build in snacks and drinks, only an active sim could upgrade training equipment to be more challenging, and a clumsy sim could maybe be able to make some stuff by accident sort of, but unable to replicate it at will.
Heck whenever a clumsy sim makes a potion or such there could be a small chance it ends up being something else instead.

For erratic sims, they should not be able to make any specific meal, potion, drink or such. It could always just be random. You could just select 'make breakfast' 'make lunch' or such instead. No sub-menus.

Better looking food. Not exactly sure why the food in 4's as poor quality as it is, especially when Sims 2 and 3 (2 especially) had much more detailed food by comparison.

Settings for occults.
That way I could e.g. enjoy some hyper aggressive vampire break-ins or Spellcasters autonomously swinging spells without the realism players losing their minds. As it is now, you basically never even see occults behaving like occults unless you control them yourself (or use mods). Options and toggles are something they should have had from the beginning so pack features don't need to be nerfed to nothing because some players chose to buy an occult pack and then whine when there's occults acting like occults but, alas, I don't think this is ever going to happen. At least they added a toggle to werewolves but they should have options like this for far more.

Occults updated for children.
So I like have a family with 2 adult vampires with a hissing problem... and every single one of their kids hate them because of it. The kids are vampires themselves but the game basically treats them as "normal" so I then have to deal with this idiocy. The way occult kids are now I find very disappointing and lazy.

Ability to smash their stupid phones.
Well, not exactly lol. But I mean, I'd really like an option to disable them. Truly disable them on an individual basis. Currently you can silence them but it basically does nothing. I have been thinking of getting mods but I don't like the "all or nothing" approach because the phone addiction IS appropriate for some sims of mine. (Especially when celebs can even have a quirk like that.)

Ability to edit worlds.
These empty set dressing worlds would be less empty if we could actually add stuff for our sims to do.

Customizable seasons.
So hey maybe I want to make my sims even more miserable and play a homeless challenge in a world with perpetual winter.

NPC-Households (esp. families) visiting a Community lot together.

Return of the Sims 2 aspiration meter and long term relationship bar.

Making out on beds.

A realistic job application process.

Strollers.

A lot type that automatically changes appearance for each Season.

I'd actually like a "story-making" pack focused on gameplay. I liked StrangerVille which was a bit like that. I'd like more options for some mid-game prompts that give optional things to do but I'd like that to help drive the story for certain generations or households. I can come up with stuff on my own and refer to what is in the game but I wouldn't mind a pack or concepts that add to that mid-game. I think I'm going to use some of the scenario ideas, for example, in my ongoing rotation.

Color Wheel for all CAS & Build Mode.

When thinking about The Sims 2, I really miss that my Sims can't paint my own pictures in-game. My most important character is very artsy, and he loves to paint all the time. In my old game, he painted all my pictures and had those hanging on all of his walls. I am really missing that kind of personal touch the game used to have. So saying this, of course being able to create all these worlds by ourselves is also very, but very missing.

Controllable pets
More musical instruments, to facilitate having a band
Better/more romantic interactions
The Smustle! and other dance styles