Romantic suspense, chick-lit, and vampire fiction all have one thing in common: they're powerful trends that changed the course of many a career. Predicting trends in the industry is like playing a stock market on Thorazine; you have to think two years ahead before you invest.
Take the micro-trends in various types of historical fiction over the last couple of years. I think they're tremors, gradually building up to something bigger. I also think the reason that they haven't yet is because most historical fiction is not written to appeal to the mass market, or be accessible to the average reader. There are also some story elements missing. Think of how well Dan Brown has done simply by following the basic story line of Raiders of the Lost Ark in The Da Vinci Code (not that I'm accusing him of plagiarism. I know I'd have to stand in line.)
I don't think a writer should write for a trend, but they bear watching in the event you have something that will fit into a trend. It's also fun to try to predict what readers will want down the road (because you know if I'm right about historical fiction, I'm going to rub it in big time.)
What do you guys see as the potential trend-makers in the next couple of years?