I've never subscribed to that theory. If you try to write something you don't love, just because it's popular, your prose is bound to suffer. Instead, I think you need to write what makes your heart beat faster and time whiz by when you're working on your novel.
But...and this is a big but...once you know what you want to write, you have to determine what genre your story is, and then study your audience. What are the conventions for that genre? What will readers expect? What do you have to include, and what can you get away with when stretching the boundaries?
For instance, a romance novel can have a female/male couple, a female/female couple, or a male/male couple. They can live anywhere, work at any job. But the couple must be together by book's end...in other words, there must be an HEA, or Happily Ever After.
Today's Tarot deck is the Universal Waite Tarot. Here are a couple of sample cards from this deck.
Images courtesy of Aeclectic Tarot
I asked what I needed to know about my audience for the current work in progress. The card I drew was the Two of Wands. This card tells me my audience is mature, bold and courageous. This group has clear-cut goals and needs, and they won't hesitate to pursue them. They will expect to see a heroine and hero with these same strong suits, and won't have patience for a wilting flower. The characters may make mistakes, but they will charge ahead and eventually succeed.