Witches.
Mermaids would be limited. They will need to code a world specifically for them and cannot go back to recode the water in the worlds we already have. At the rate we're going, we won't be get many more Occult Sims. I would rather they concentrate on those that can be used universally.
I'd love to see Mermaids as an interactable NPC that could give gifts or be downright nasty to Sims in a beach vacation pack. Maybe Mermaid could also be a temporary lifestate so that your Sims could at least experience being one.