Ooo, this thread has really got me thinking! My wish-list for future Sims 3 expansions/stuff packs:
Seasons & Weather - really loved the snow in Sims 2.
Pets - but, as other posters have said, not just cats & dogs, is there any reason we can't have horses? A mad scientist could even do a Jurassic Park & clone a dinosaur for his/her kids! Just think, you could go out and collect amber washed up on the beach, and have the option to extract DNA...
Hobbies - but make it so we can choose not to have a particular hobby please, in Sims 2 my sims would always ended up having the cooking hobby because that would be the first thing they did that had a hobby attached. Grr!
Superstar & Makin' Magic from Sims 1 - I enjoyed these expansions but I never seemed to get very far in the career tracks with them and felt like I'd missed a lot (they got neglected when Sims 2 appeared, of course!), I would love to have something similar in Sims 3.
More World Adventures please! - I just think this is the best expansion pack EVER! And there is so much more you could do! My current top 3 destinations are: Mayan/Aztec (insert culture of choice) ruins in a South American rainforest, Italy for Rome to explore Roman ruins & those catacombs full of skulls under the city, and Thailand or similar (Simkor Wat anyone?!). I am also considering what you could do with Iceland (Vikings & Volcanoes!), a Greek Island (sorta similar to Rome but how about visiting a cave which leads to Hades, paying Charon to cross the Styx & feeding a sop to Cerberus? Hmmm!) and an African country like Botswana or Kenya for a safari (a reward in which you can take an elephant home would be awesome!).
Career-wise, I'm waiting to see what Ambitions is going to include. There is still a lot of things that need to be ironed out with Sims 3 (anyone else had the Invisible Maid service? and the thought-bubble on the ground which turns out to be a friend who is now either invisible or only an inch tall?!), but I'm sure it's going to keep getting better!