Would love for them to revamp mermaids! I'd want more CAS options to make them look less human-like and more features of different underwater creatures, and since we can now paint fairy wings, why not let us fully customize mermaids' tails and fins?
Also, more traits and aspirations related to them, such as "hates land sims" or a siren trait. It would be nice to have traits that lead our mermaid sims into being good or evil, and some could just be neutral. I'd also like fewer rabbit holes and more playable features that we can do under the water, like exploring coral reefs and shipwrecks or teasing fishermen by pulling on their hooks so they think they've caught a fish.
SIRENS! Both CAS and build features just for them. They could add so many things, and yet they aren't, or they just don't care.