"Write what you care about and understand. Writers should never try to outguess the marketplace in search of a salable idea; the simple truth is that all good books will eventually find a publisher if the writer tries hard enough, and a central secret to writing a good book is to write on that people like you will enjoy."
~Richard North Patterson
I think this is good advice. It's impossible to outguess the marketplace. By the time you write and submit book with a "hot" subject, the editors have already moved on and are onto the next "hot" subject. Everything circles eventually, so don't worry. Write the book you need to write, the book you have to write, and make it the best you can. After all, your name is going on it.