Here is the answer from Felicia Eth, founder, Felicia Eth. Literary Representation:
It's always temping to write something that seems trendy. Much of this business is about the selling aspect, so writers often think that if they write what publishers seem to be publishing, or what seems to be appearing on bestseller lists, then they have a greater chance of getting a contract--but I honestly don't think it's the wisest way to go. Sure, a writer needs to be aware of what's out there, both so you're not reinventing the wheel (i.e., writing a book that's essentially already been published), as well as so you know how to position your book--but you really need to write what you write best. This means if you've never written for the YA market and have little sense of that audience, then starting now probably doesn't make sense, nor for that matter does reinventing yourself as a political writer if you don't already have a column or blog that's well known in that arena. You're not likely to "fool" publishers simply by trying to do what's hot.
Beyond this though, there's the issue of timing. This is a business where things move slowly, so whatever it is you'd be selling today most likely won't come out for a year to two-and-a-half years. Who know where the market will be at that point? Being conscious of the market is key--but above all, your book needs to be the best it can be. Who knows--maybe it it's terrific, you'll start the next trend
This gives me hope, and I've always watched trends and cycles in the literary world. I think everyone has their 'time', obviously Agents can tell this better than writers ourselves. Just because we receive numerous rejections doesn't mean we're bad writers, our books may simply not be ready for the market at the moment. Patience is a virtue, but also torture. So I'll wait.