Every year there are new books published with faeries, mermaids, ponies, dragons, and dinosaurs. And ninjas. And pirates... They sell. Of course, there are probably lots of people writing about them, too, so it's always good when you can take a popular theme and do something new with it. For example, DANGEROUSLY EVER AFTER is my all time favorite princess book about a princess who loves dangerous plants. You can even take traditional stories and turn them a little bit sideways, like with Mark Teague's THe THREE LITTLE PIGS AND THE SOMEWHAT BAD WOLF (he suffers from low blood sugar). Basically, there's still room out there for a good mermaid story, but you'll want to make sure it stands out from the crowd.