The general advice out there is "don't write to market". And when I say that, your response is, "what the heck are you talking about?"
I'm ahead of myself here.
All right, so, imagine if you will all of a sudden this book comes out where an attractive teenager starts a torrid romance with, I don't know, a were-dolphin. That's a thing, right? And it goes huge. The kids are going crazy, getting tattoos and going to conventions with flippers and they're talking movies and everyone has Blowhole fever.
And suddenly were-dolphins are a THING. Other publishers are putting out their were-dolphin books, and someone else is trying were-shark thing, because that's what that third-leg of the love triangle was. Agents are all, "WHO HAS WERE-DOLPHIN BOOKS QUERY ME NOW!"
So you say to yourself, "I should write a were-dolphin book because that is HUGE and it'll be bought up and sell like gangbusters".
Except no, that's a terrible idea.
Even presuming you can crank out a were-dolphin book in a month*, and get it out to agents like BAM... it probably wouldn't matter. Because the publishing industry is really not geared to respond to trends. "Glaciers honk at the publishing industry to move faster". A super fast-tracked novel from writing-to-agent-to-publisher-to-bookstore would still take a year. By which time everyone is bored to tears with weredolphins and have moved on to sexy steampunk airship captains. Do you have a sexy steampunk airship captain? No? Then no one is buying your lame weredolphin latecomer.
Do not write to the trends of the market.
I honestly think the best "what should I write" advice is this: write the books you want to read. Write to the market of the book you want to buy. And count on the fact that you have fantastic taste that everyone else will want to read it as well.
Next weekend I'm at ConDFW in Dallas. If you're in the area, come check it out.
*- Or have an manuscript that you can find-and-replace all your elder-god-in-teenage-body references for were-dolphins. Or something.