Agreeing with a lot of the already-mentioned, but have to first flail about bunnies. I'd love a Small Critter Pack as pets: bunnies, hamsters, guinea pigs, chinchillas, ferrets, rats, mice, galagos, all things small and fuzzy and cute. Not just in cages, that becomes a meaningless chore of feed-clean-object, but living freely in your apartment like cats and dogs do. That said, I absolutely approve of the decision to handle just one or two things per pack so they can be given focus and depth they deserve. I'm also all for a farm pack, or horse pack with racing or other in-depth additions.
As for supernatural lifestates, can't wait to see what they'll bring out next! I'm sure they'll bring out werewolves, fairies, witches and mermaids eventually, and with lovely in-depth full-focus packs, too.
Timetraveling pack! Possibly several, so enough focus can be given to the worlds they end up in: historical world/s and futuristic ones.
More vacation worlds! Camping is all good and fun, but surely the sims could use a holiday somewhere else, too. World Adventures. WORLD ADVENTURES. All the locations from it, or similar, and definitely adventuring mechanics. Also a skiing holiday world, and scuba diving island paradise kind of thing.
Seasons. Personally I'd make them free download like toddlers were, all across the worlds and neighborhoods, and then make a ton of fun season related packs, like winter holiday pack, summer picnic pack, autumn harvest pack and so forth, but I'll happily pay for a Seasons pack too. Just thinking it'd make most sense for them to make seasons a base game default system.
I also liked these from the earlier suggestions: university, interior design career, slowdancing together, spiral stairs, ladders, table top RPGs, sleeping bags, ballet/tap/jazz dancing, theaters, actor career, teacher career.
I also loved someone's suggestion of Elders' pack. Elders need love and attention and more focus, too!
Also more in-depth things with photography: ability to take pictures of whole family, as someone mentioned, bigger group photos, included toddlers and pets. Pretty please for more interesting frame options, collages, corkboards and the like for the photos the sims take. Albums to put the rest of the photos in so they don't have to be in sim or household inventory, or covering the whole house's walls.
I would also love it if the books the sims write became purchasable for other sims (and the household itself). Now if you want to have the book the sim wrote in their shelf, need to write it twice.
Would also love it if the club activities got updated with things like: play with pets, train pets, be friendly to pets, go on walks with dogs, and so forth. Also would like additional filter to club requirements with Skill +5, or Skill +7 for the really masters of their trade clubs. It would also be useful if the career requirement would show and separate the options, so chef and mixologist, musician and entertainer and so forth.