A huge hope I have is for sims that are unique in their personalities, situations and moods
Starting with personality creation. I'm thinking it would be great to have a selection of base traits - the core ones everyone has - like tidy/messy, friendly/mean or introverted/outgoing that could be set on a scale. Neutral in the middle and each extreme either side, so we can select the strength of the trait. You could have a slightly mean sim, or an extremely mean sim, for example. This gives a lot more variety to personalities. Additional to this, some number of non-linear traits could be selected for things like being good with tech, loves dogs, doesn't like children, or clumsy, etc.
This kind of personality building would be great, but it could be hugely better with life traits that are aquired through gameplay. That way each sim changes as we play. Life events, good and bad, should add something with a long term effect.
In fact, some should be permanent. Berevement could give a trait that stays with a sim permanently. Its influence could be changeable through gameplay, for example it could be healed at some point, but I'd like to see it treated as a permanent influence on their personality. Even if it's healed, they'll still have gained something long term from it, for example being able to talk about it with other sims.
Some could be developed by number of events. For example they've flirted sucessfully with a certain number of sims, they could gain a trait that makes them better at it. I like the idea of gameplay outcomes giving bonuses - or negatives, if they happen enough times. A sim who often fails at romance could get a negative trait that makes them worse at it. It could be lifted in time by doing it well a certain number of times. This kind of thing means we'd have to try harder in gameplay to overcome difficulties- no more spamming actions, and gives some element of risk too. Maybe something like an 8 out of 10 rule. If there's eight unsucessful interactions in the last ten tries, you get the negative trait. I wouldn't mind if these kinds of traits stacked either. You could end up with a sim who is incredibly good at telling jokes, or one who is incredibly bad at romance interactions.
Some traits could be long term, but eventually go. A sim who graduates with a high mark could gain a condifence trait that helps with advancing at work, and it lasts for a few years of their life. A sim who gets fired from work might get a trait that makes them less confident and perform worse at work for some time.
Others should stay for as long as a situation is true. A family oriented sim could have a positive trait for as long they're married. A materialistic sim could have a positive trait for as long as their net wealth is above a certain level. A sim who hates children could be 'frazzled from baby' for as long as there's a baby in the household. A romantic sim without any romantic relationships could have an unhappy trait.
Short term moodlets are totally ok too, they're realistic to life. I just think they only go so far in representing a personality and mood.
I'd like to be able to select a sim and see in a menu what traits are contributing to what, basically how their personality and life situation makes them feel. I don't want any single trait to be overpowered, just for a selection of them to combine to make a whole picture.
Also, although it's not an attraction system, it would make for interesting gameplay because some traits would make it difficult for sims to live together and both be happy. A materialistic sim might be unhappy in a modest, eco house, while a free-spirted sim might hate living in a giant, expensive mansion. You'd get lifestlye compatibility for free with this kind of system.
Anyway, I'm super curious what kind of personality, life and mood features other people want to happen?