Except your own. Writing to the market always falls short of the mark. Besides being a soul-numbing experience (because you end up essentially writing someone else’s inspiration), it cannot be researched sufficiently, drafted, rewritten, edited, rewritten again, shopped, edited, and published in time to capitalize on the market trend. So, you have invested valuable time, energy, and effort in a project to which you are less than 100% committed, and about which you are less than passionate.
Start with what you want to read. Do what you think is right. Draft your concept. Outline it, write a few chapters and share it with someone whose skill, perspective, judgment, interests, and discernment you respect. Odds are that those pages will jump to life in the reader’s mind because you care, because you’re invested in something you want to say, in a tale you want to tell.
Trying to forecast the market, or read editors’ or agents' minds wastes your time. It also paralyzes your writer’s instrument. The skills that you develop as a writer are important, high performance, precision tools. Don’t use your scalpel as a screwdriver. Don’t use your best sagacious voice to make someone else’s hero sound interesting. Respect yourself, your ideas, and your time. Follow your muse, your heart, and craft the stories you think matter, the ideas, subjects, and characters that wake you at 3:00 am.