Two more ideas I have kicking around in the back of my head that I may or may not write eventually:
(1) A Pride and Prejudice/Little Mermaid retelling mashup, with Lydia, the youngest, impulsive Bennett daughter in the role of the Little Mermaid
(2) A series of books called Six Sisters of the Sea, about six human sisters who have a family secret: they can turn into mermaids at night. Each book would focus on one of the sisters and be a loose retelling of a popular fairy tale.