An overhaul of the traits system currently in place. Either:
1. Bring back the point-system in TS1 and TS2. Their personality has to be balanced, and being really good at one thing comes at the expense of being really bad in another.
2. Keep the current system, but certain traits are flagged as "good" or "bad." After a few good traits, you need to pick a flaw.
Either of these would make for more interesting characters. Yeah, it would probably have to come into fruition for TS4, but the ability to create perfect sims needs to be crushed mightily once TS3 runs its course.
I like the trait system, but I want to bring back the point system, but used as traits. So parents can praise and scold their children for pulling pranks, fighting, etc., and it could cause influence on what trait they will have later in life. Maybe Sims can simply gain or lose traits too, the way Pets do. Or they can encourage their children to be as evil, or good, or as nice, or mean as they want.
The only reason why I kind of want the closed neighborhood back, is to open up the restaurants, and stop the bloat, from playing multiple households, which I like to do.
In that case, speaking of Evil Sims. They're really fun to play with! Hetty Lionheart is my favorite Sim, and you know what trait she has? Evil! I think children should be meaner, and have more mean interactions under the mean pie menu! They need to back talk their parents once in a while. I'd like to see Sims start food fights as well!