Some ideas:
Holidays (Christmas, Halloween, Etc.)
Hotels like in Sims 2 Bon Voyage
I think it would be cool if your friend could like bring their Sim into your game (Like Online-ish play, but only if you want to.)
I also think it would be kind of cool/funny if Sims could get drunk.
Seasons, Rain, Snow (this is probably going to be an expansion pack coming up)
Private school
More realistic aging (Like Adults' hair starts graying towards being an Elder, Acne for teens)
Improvements to things already in game:
I wish the Weddings and Funerals were more realistic (Like the preacher person or whatever at Weddings and the Funerals being less like parties)
I wish that if people changed neighborhoods, they would keep their old relationships and they could like call their friends and their friends could come visit