More traits would be good (like cowardice and bravery, as well as fun ones like Never Nude), but I really want more options for creating family relationships. In addition to having engaged and dating sims, it would be nice to be able to create divorced/ blended families with step-children, grandparents raising grandchildren, aunts, uncles and cousins, and to edit your own family trees.
It might also be nice, if they ever give us a memory system like in TS3 and TS2, to be able to give your sims key positive and negative memories that might affect their personalities and moods. (Like a good memory of getting A in school or a bad memory of a parent dying). With that, we could get customizable likes or dislikes or interests (if we ever get hobbies). I also miss the ability to give sims a favourite food and style of music (although I freely concede that this last one is not that important)