I think I would like them to build on what's already there. Focus on the transitions between emotions (like Meaningful Stories does), so Sims don't fall from one emotion into another all the time. Make anger linger and sadness last longer, and choose which buffs weigh the heaviest (a well decorated house shouldn't outweigh the loss of a loved one etc).
Also a whims overhaul (whims that correspond with traits) should be a priority imo. I think they can really inspire gameplay and add some quirkiness. I'd love it if my childish Sim would get a whim to 'draw on placemat' while dining out. These kinds of details matter and make the Sim's personalities more believable.
Taking a look at the idle animations would also help. Sims all seem to share the same smile. If they'd add three different idle facial expressions it'd already help for livening up the game and the personalities.
