If you could pick ONE existing career to become a playable active career…

Culinary is one that should have been an active career, so it gets my vote. After that entertainer.

I'm almost glad the others are not actives, doing them right would take too much resources and for some like business I can't imagine the effort being worth it. Others like Athlete would basically mean making a whole sports game within Sims, and Astronaut a whole space exploration game. Too much for too little.