One of the big buzzwords with Harlequin editorial is unpredictability. However, it can be hard to assess what that means. What it doesn't mean is an end to HEA or indeed the books suddenly not being a romance.
My personal take on this is that is akin to Page Turning Quality and holding the reader within the story. If a story becomes dull and predictable, the reader is more apt to put it down. If she is totally drawn into the world of the story and finds it difficult to guess what is going to happen next and therefore has to keep turning the pages, then the story be definition is a Page Turner. If you can keep the reader (and in this case the hard-boiled editor) turning pages and perhaps even missing a meeting or staying at work late, you will have conquered unpredictability.
How to make the story unpredictable -- it starts with the characters and making sure that they are fully developed and well-rounded. Make sure you really know their personal values and how that translates on the page. In other words, how is the reader going to be super-engaged with the characters? Why is she going to identify with those characters and their journey, and why is that journey going to be different than the 1000 other romances the reader has read?
Characters who are 3 d and who are placed in situations where they have to grow and continually challenged will give the reader unpredictability.
Long ago, there used to be an idea that Harlequin was written with really tight parameters. By page 6 a kiss and exactly page 128 you had to have some sort of sexual encounter while on page 157, not 158 or 152, they had to have a fight. Ummm, hopefully you can see the idea was false in the extreme and has led many unpublished authors to have more manuscripts rejected. There has NEVER been a precise formula. Ever.
The only thing that is sacred is that the focus is on the growth of the emotional relationship of the couple AND that there is a HEA with a closed story line. In other words, the reader is not left wondering if this couple will be fine. (Very occasionally, Harlequin has done a 2 part story line but these are well-flagged in advance and many readers can't read the books until they have both parts!)
But other than those two bits, the journey and the manner of the journey is up to the writer. It is dictated by the characters and their reactions to specific events. The line is determined by the type of story that the author pens. For example, all historicals are set in the defined past. Modern/Presents have an escapist glamour about them etc.
So you want to inject unpredictability in to your story? Make sure you start with two strong well rounded characters who are forced to challenge their core beliefs and see where that leads you. And when you are developing the characters, think beyond the cliche and develop the sort of characters which attract you. Who do you want to write about? What are you passionate about? Why will the hero sweep the heroine off her feet? What is about him that makes him a hero in her eyes? Why are these unique characters? Why will the reader want to read about them? How are you going to show their uniqueness on the page?