For me, a book idea starts off with a single sentence that can be quite broad. Two exes stuck together on a cruise. A wedding planner forced to work with someone who hates romance. Not a tag line, per se, but a general description. Is it enough to build an entire book upon - of course not!
As much as I love romance, two people falling in love is not enough to flesh out 98,000 words. So we add in layers - the stories within the story. A great place to begin is with secondary characters. Do they have their own journey? Their own romance? A romantic suspense automatically has this covered: romance and the ongoing action/suspense split the plot load about 50/50.
I recently finished Winter Garden by Kristin Hannah. Wonderful book, and it started me down this road, due to a major character encapsulating a completely separate story within the main plot. Sure, there are several ties from it to the overall plot, but it is truly a stand-alone.
My first inclination is to lean on the secondary characters, make them pull their own weight. Give them their own arc. Throwing in an additional romance is an easy way. Readers feel as if they're getting double their money's worth! Add some suspense, and suddenly there are three stories intertwining. It becomes a bit of a maze to the writer, but done right, will create a multi-dimensional experience for the reader. My current WIP has an ongoing reality show as the mini-story. It is a departure for me, since I usually add a secondary love story. What do you prefer to see?