I am working on a book for my kids (ages 10, 13) they love Harry Potter and similar books. I am writing a story about a private investigator that lives in a modern day but deals with various different fantasy elements like Orcs, vampires, witches, ECT. He eventually meets beautiful woman performing in opera that happens to be a Siren. The PI believes she is trouble but ends up having to save her. How do I get to the point of her being possible suspect then to being in need of saving? And hints of how the two falling in love in a teen/young reader story?
I'm not sure anyone could give you those ideas without it becoming their idea for a story. You should develop your own idea for these things. You could try thinking about all the various hero saves a girl type stories and alter something enough to make it your own concept. There's thousands of books, movies, TV shows, and games with a guy coming to the rescue of some fair maiden. I don't mean to say that in a bad way. The theme has been done so many times because guys like to feel like being a hero and women like (I'm gonna get yelled at for this.) ... Women like the idea of a mysterious handsome stranger coming to their rescue. Try finding some of the stories that have been done and alter them enough to make something new. You might also want to take a read of one of the Harry Dresden books by Jim Butcher, it has similar themes. http://www.jim-butcher.com/books/dresden