Many have already mentioned baby-toddler-children interactions, new instruments, hot tubs, seasons, new careers, etc, but I have to remind you all of the most important and immediate thing:
We need pets!
(At least the most important to me.)
Even though some of you don't like it, the pet expansions are in my opinion the best. First let it be clear that the main reason it is always one of the last few expansion packs is because of the improvement it brings to the game. When the game starts going down after a few boring expansions and loses some popularity (as what happened with TS1 and TS2), they give us a Pet expansion.
Now on to what should be in it. Clearly, the same old dog-cat-hamster-bird content isn't enough.
I'm not sure where some people think we could just leave a horse or dinosaur at. Maybe if there were some new rabbit-holes where you could buy/get/raise a horse, like a racing track, or give money to the science facility for them to research some DNA stuff and eventually make a dinosaur. Either way, the animal itself wouldn't be shown - it would be only in the text, or maybe have a picture like the insects do now. The top of it for dinosaurs would be having an exhibit in which they get tinified (like the aquarium).
Dogs and cats should of course be the topmost customisable content, with traits, careers, skills and clothing of their own (Yes, clothing. Maybe a part of beauty contests or some kind of pet model career.), but let's not forget about the others. Why is it so that Sims never had snake pets? People in real life do. Maybe spiders. Iguanas are obvious. Oh, and EA staff, in case you ever read this, remember: If you're gonna add guinea pigs, make it so they can have bigger cages, with those maze-looking tubes, and more than just 1 per cage.
Sorry for the long post, but I underlined the main stuff in case you are too lazy to read it all. =P