When I played The Sims 2 I liked working on the floral bench and creating bouquets. Then after creating the ones that replenished the need bars I would create a spa business with them, one of the styling chairs (to alter the appearance of sims coming in) and a massage table. I would love to be able to do that in The Sims 4. With all the things they have added for the Wellness skill there would be a lot more interactions possible - especially with the manicure/pedicure chair, the yoga mats and the meditation stool. Unfortunately we can't really create a business as it stands now doing things of that nature.
I really liked those Sims 2 work benches for making toys, robots, flowers, clothing and the Sims 3 Alchemy table etc. I would love to see them come back at least. Creating those things would be careers in themselves and would give even more use to the things we harvest and collect.
As for straight up careers - I find it odd that the education career track is not in yet. I have no doubts that one of the expansions in the future will be college or university so I am thinking maybe they will release it then.
Expanding on the artistic / entertainment branches would be good - magician, actor, musician, dancer, singer (both pop and opera as someone said earlier), juggler, dj etc.
I think they should revisit the list of past careers available in the franchise and introduce them into this incarnation of the Sims. The careers are good - I like how interactive they are compared to past ones but I would love to see even more choices available even if it's just send them off to work and have them work on projects in their off time.
I also agree about a veterinarian career, that was a great suggestion made by a few people.