I want witches most (I've been dying to get them. Every one of my sims in Sims 3 was usually a witch because it was so fun for me)
But this thread got me thinking about how mermaids would work and how to make the best of it. An island world (with a bay hopefully, because that would be cool too) where the lots are on the coast. You could ride boats on the surface of the water, play in the beach, but below the surface would be open space on the ocean floor for mermaids to swim around and interact, with lots there too to design underwater homes or "parks" or something. That would be cool.
But I really am dying for witches