I like the idea of a Farm/Ranch expansion pack. Complete with chicken coop, milkable cows, horses, and pigs that will eat your spoiled food scraps.
Another idea I have is that of a Circus/Carnival expansion pack. They could include lions, tigers, elephants, and trained seals, and circus performing acts. They probably won't do this because of controversy over use of wild animals in circuses though.
I also like the idea of a Hot Date/Nightlife/Late Night expansion pack. With restaurants (fast food, diners, ethnic food and fine dining), bowling alleys, movie theatres, arcades, dj/dance clubs, and even amusement parks perhaps. Or Amusement Parks might be its own expansion pack/game pack (objects like the ferris wheel and carousel are likely to be time consuming to make).
University/University Life should definitely make a return. I like the abundance of majors in Sims 2, but I like that adult and elder Sims could go in Sims 3, not just young adults.
Generations should make a return for sure, and I think this is where they should introduce toddlers. They could even call the expansion pack Toddlers for all I care. All the stuff for toddlers should be included, like strollers, playpens, high chairs, and baby swings, plus they should include family activities like scrapbooking, teaching children to help cook/bake, and possibly sewing. Sewing could otherwise be included in a later Hobbies expansion pack, which could include sports, pottery, fixing up junker cars, dance lessons, and competition between Sims.
Seasons is another must-have that I think they will do eventually. I really miss having weather and seasonal festivals in the game.