There's a lot of stuff they could add just to fill in the gaps in existing traits- emotional states without a corresponding trait, existing traits without a clear thematic opposite, etc. Just for some examples:
Emotional
Absent-Minded: Conflicts with Creative and Genius. These Sims seldom get Inspired or Focused. However, they seldom get Bored or Tense, either.
Lethargic: Conflicts with Active. These Sims seldom get Energized and become fatigued more quickly from physical activities (split from the Lazy Lifestyle Trait).
Timid: Conflicts with Self-Assured. These Sims tend to be Embarrassed and seldom get Confident.
Lifestyle
Carefree: Conflicts with Ambitious and Perfectionist. These Sims get far fewer negative Moodlets from career failures, crafting low quality items, and losing in competitions, and may become Tense if promoted too much.
Cautious: Conflicts with Clumsy. These Sims tend to fail less at physical activities, take longer to complete them, and are more likely to become upset by failure.
Frugal: Conflicts with Materialistic. These Sims become Tense when spending too much money, become Happy when using cheap objects, and can Brag about Saving Money to other Sims.
Hates Animals: Conflicts with Cat Lover and Dog Lover. These Sims become Tense when around animals and gain relationships slower with cats and dogs than the average Sim.
Indoor Person: Conflicts with Loves Outdoors. These Sims become Tense when outdoors for to long, become Happy when first going indoors after a long time spent outdoors, and gain powerful Moodlets when inside during bad weather.
Social
Kind: Conflicts with Evil and Mean. These Sims become Happy when they Donate to Charity or help other Sims, and can Cheer Up other Sims to reduce their negative Moodlets.
Humble: Conflicts with Self-Absorbed. These Sims may become Embarrassed when receiving too much positive attention.
Trusting: Conflicts with Jealous. These Sims build relationships faster with other Sims, but gain powerful negative Moodlets if they experience cheating or suffer Mean or Mischievous interactions.