h2o and the originals share cast and people have used that connection to make stories, here's my take.
Hayley is Cleo, Emma is a doppleganer because it makes more sense than Rebekah playing the role of fellow mermaid.
Hope mikaelson living away from her family for her own safety. After her family finally shows up, and decides to become a part of her life after all these years. Does Hope let them, or does she take on what she does not understand by herself. Will they learn to trust each other, or will it be a never ending battle?
Side note: I do not own the originals, characters, or any familiar plot lines. All those credits go to the creators and writers of the show. I only own my characters and plot.