What I'd really like to see is more complicated personalities, and maybe sub aspirations. For example, two types of romance sims: a promiscuous romance aspiration like what we have now, and a true love aspiration where the sim is terrified of affairs, wants to be with one sim their whole life, etc. This could work for knowledge too; you could have a sim who's all about supernatural knowledge, or a sim who's all about book knowledge. I also really want some kind of do-gooder aspiration, where the sim just wants to help people. Not sure how that would work, though.
I also would like to see chemistry being a bigger part of the game and based on more factors, and I'd like to see music tastes vary from one sim or another. (Lilith Pleasant might like metal, but Angela prefers New Age.) I'd also like more parts of personality; I think that creativity and logic should be personality traits, not skills. So for example, a creative person will have an easier time learning things like playing piano, painting, writing, etc., but all of those are individual skills. Just because a sim can write a great book doesn't mean they can paint! Sims who are inherently logical would have an easier time learning how to fix things, how to play chess, etc., but they'd still have to build those skills.
Basically I just want my sims' brains to be a lot more complicated.