Culture Expansion pack, please!
I find that a great pack expanding in career related activities outside home would be great. In sims 3, we could attend games and concerts and symphonies as well as participating in festival. I think adding these options to Sims 4 would be amazing.
If you pursue a career in music or comedy, they could be linked to these events happening in town (You could book a venue for perfoming and earn off from ticket sale) and you could also attend these events, Open mic nights for comedian career, band playoffs for mucicians, book signing events for writer career etc.
Additionally - A lot of careers in Sims 4 become really boring after a while, because they don't offer anything new to the game. Adding a playable mode to already existing careers would make it more enjoyable (firefigter career in sims 3). I find the small business pack was a great start, but careers that are already in the game will then mostly be not used, because they just bring in money and nothing else really.