I was quite looking forward to traits in sims3, only to find out it usually has very little meaning. Heck, sims 2 sims have far more personality without the traits. That just shows they didnt do it right.

A trait shouldnt just affect a few interactions or a few things your sim likes to do or is better at, it should help define them as a person.

Let's use a few fail traits from sims 3 as an example, for how they should be put in sims4 if making a return:

-Daredevil.
Eating dirt is the only thing they can do, interactions get named differently yet look the same. One of the most disappointing traits.
No these sims should really look to live on the edge.
At a pool they would love to do dangerous jumps from a diving board or just from the side of the pool, they would do a wheelie as they drive off on their bike, try and do daring things when playing sports, as kids they'd try to impress other kids on the playground by hanging upside monkey bars and such, etc.
Things should never be dull for a daredevil sim, they want exitement and should actively look for it.

-Evil.
Another big disappointment in sims3. The evil laugh was nice, stealing candy from a baby was just sad.
They should actually have an evil slumber, sleeping with a dark grin on their face.
Any job they do, they would be able to do it in a corrupt way. There would be a chance they'd get caught of course and would be send to jail.
Befriending other evil sims should be easier, having the same goals and making evil sims less lonely.
They could have the desire to transform other sims into inhuman sims, break up relationships of other sims, and otherwise be a real menace to others.

-Insane.
Insane sims should be well.. insane. Not just a little quirky but insane.
Their wants and fears should be completely random and silly. They could want to go fishing with their dead father, eat steak in the morning, sleep under their bed, hug a complete stranger and they might fear crying of a baby, sitting on a specific chair, socialising with a specific sim, etc. What they fear could be different each day.
They could also have weird interactions no one else can do such as treating stuffed animals like a life pet, splashing wildly around while bathing, sitting upside down in a chair, making unique meals that they enjoy but most other sims would find gross due to the odd combination of ingredients, etc.

-Cursed.
Wasnt so bad in sims 3.
Cursed sims should be well.. really cursed. Whenever there is a risk of death, turning into a mummy or other misfortune happening, it should happen to them. They'd be leading either sheltered lives or short lives.
However, there should be ways for a cursed sim to lift the curse. Other sims could help with this.
The cursed sim could have to carry a specific crafted item in the inventory while exploring a catacomb with another sim that has a specific trait. The cursed sim could be required to have a specific skill maxed out as well.
This could allow the cursed trait to be turned into the blessed trait that could only be available in this way, as an upgrade to cursed.
Blessed sims could die pretty much only of old age and could survive electrocution, falling meteors, fire and everything thats not old age.

-Clumbsy.
Didnt do much either in sims3.
I'd like clumbsy sims to truly be clumbsy. They'd step on toes when dancing with another sim, trip from stairs occasionly, be the only kind of sim to have trouble opening drawers, and could occasionly do something wrong at work. They could even break items that no other sim manages to break such as a fitness machine.

-Genius.
This shouldnt just allow a sim to increase the logic skill faster, they should truly be a well.. genius. Their grades in school should be high without trying, they wouldnt have homework since they already finished it in school, all traits are improved faster through books but at a normal pace otherwise.
Socially they'd have some issues though since other sims might get jalous of their intellect and genius sims may at times be too smart for their own good and insult others without knowing it while merely stating facts.

-Virtuoso.
Not only should sims like this master instruments much faster, they should be able to write lyrics.
I'd actually like for any instruments to require teaching first, visiting a lesson or hiring a tutor before sims can learn to improve themselves on an instrument. the virtuoso sim wouldnt need to be taught and could get to work on it right away, teaching themselves as they go along.

-Good.
Quite a basic trait, but it should be more ambitious. Good sims should be able to do volunteers work around town at various places, allowing them to meet new people and make friends while at it.
At their jobs they could also have unique options available such as holding a goodwill concert as a rock star, donate money as a business director or fight corruption as a politician.

-Handy.
Instead of just mastering the technical skill faster, it should be mastered faster through practical use and at the same pace with books. Being handy says nothing about study ability after all.
Handy sims should also be able to do technical things that no other sims can do at level 10. They'd be able to upgrade objects in unique ways.
Simple car modifications such as adding a spoiler, unique paintjob and add a stereo could be done by all sims, but handy sims could also add custom lights and other gizmo's to it.
This skill should already have effects for children, allowing handy children to do technical things earlier than other sims.
Nothing dangerous like fixing a tv or computer. But they could fix their own bike if needed and be able to build a tree house instead of asking a teen or adult to do it for them.

-Flirty.
Flirty sims should be obvious right away. they want attention after all, it should show in the way they walk. Female sims swaying their hips a bit more and male sims walking with confidence. They should also try and make eye contact and draw attention when seeing sims that appeal to them.

-Disciplined.
Shouldnt just affect how fast a sim learns martial arts, but should show through their entire life. A disciplined parent could be more strict with their kids, wouldnt be one to lounge around much and prefers an orderly life free of chaos. If theres chaos, they'd want to put an end to it.

-Diva.
Diva sims should be more demanding. If they want yellow roses from their partner, they'd be unhappy with red roses. They'd get mad when seeing another sim wear the same outfit as them, and would get upset at any sim that triest to upstage them.
Their walk should be confident, its all about them after all.

One of the things I hated about TS3 was how they added new traits with each expansion, so we ended up with a billion different traits that are redundant or meaningless. Instead of adding new traits with each EP, I would rather see each trait become more meaningful with each EP. Add new interactions to the existing traits so that they become deeper and more potent as the amount of content grows.

It's a very shallow, unbalanced system in general. Some traits basically do nothing, while other traits cause your Sims to do the same actions over and over and over.

I dont mind traits being added, as long as all traits in the game apply to anywhere it makes sense. It should really shape a sim and not just be something that changes/adds three interactions or such. Same interactions being just named differently should be removed all together. If it looks the same, it should be the same interaction.

What I'd love is if they reduced the number of traits, but made traits behaviour dependent. Like with 'training' pets, if you have a mean spirited sim who you never let do mean interactions, she'll slowly sweeten up. Or a slob who you make clean up a lot gets the neat trait eventually. And maybe a sim gains 'skill' traits like natural cook and angler etc by earning a certain amount of skill points towards the associated skills.