I've been kind of bouncing back and forth between Sims 3 and 4 and whats become very apparent to me (is even with traits) the sims really have no unique personalities in the Sims 4 that make them stand apart from one another.
Personalities can be defined by emotions yes, but its not enough in the Sims 4 (as they work now) and honestly the emotional system is kind of annoying to manipulate especially from a story tellers (point of view) because their emotions are like some crazy roller coaster. Constantly flipping and changing from one state to another within minutes. Happy face, sad face, pouting face, straight face, happy face. Emotional states should last longer then that.
As for traits in the Sims 4, they feel like bonus or handicaps for sims, rather then defining elements of them.
So I read a few other posts and such of people complaining about the emotion system and was thinking of how it could be overhauled at this stage without requiring a complete re-write of coding. As well to keep currently players who are happy with it, happy.
My solution/suggest would be be found in create a sim, in the same way you pick a sims walk style you pick from 3 basic states for your sims emotions. Adding traits that effect their emotions to the below would further make them stand apart.
1. Non-emotional, straight faced. Your sim doesn't react to much and takes a lot to get an emotional reaction out of them.
2. Normal, similar to the current level of Sims emotions as it is now, BUT toned down just slightly so that emotional states last a little longer.
3. Highly emotional. The roller coaster of emotions is back, these sims react fast and furious to outside and inside stimuli.
One thing I'd do regardless is have emotional states last longer and not be so easily effected by player choice so quickly. Right now I get that its needed because they so easily pout, but if you get a sim into an angry mood then he should remain angry for a while. If he's happy then he should be happy for a while. Not insta-click changed. If they are in a happy mood and something slightly bad happens then its a sliding scale of happy. Not insta-pouting.
Speaking of pouting, wow like are all these sims so fragile as one single wrong choice makes them feel bad and pout about it? Like seriously do they need to express their feelings like they are talking to a therapist over every little thing that happens to them? Talk about easily triggered - except that COULD be a trait you could add. Honestly this is really the whole emotional system right now, they are SO easily triggered between emotional states that its often times annoying and more so removes any type of personality when all Sims are governed by the same system.
I also feel more needs to be done to define personalities and contrast between sims, which could relate to relationship stuff such as compatibility or heck even zodiac signs like in past sims.
I don't know if they'd bother tweaking it at this time, emotions but if they plan to keep supporting it into the next few years I feel this system needs an upgrade of polish. Especially if they want to keep evolving it like other service games do.