"Oh! YA Contemporary is the THING. I will write one right now."
"Look at all these thrillers coming out! Probably I should rewrite my current project as a thriller."
"My critique partner's friend's agent told her that MG Coming-of-Age Magicial Realism Mysteries are rushing to the top of every editor's list. I'll get right on that."
And you know what I'm going to say next: Don't do it.
The thing about trends is that they're sort of like tornadoes. You're tracking them carefully as they gain speed and momentum, and then, without warning, they change direction, tearing up everything in a completely different path.
Trend-chasing will always set you behind the curve. Why? Because by the time you've plotted and written and revised and queried your manuscript, the trend will have changed. Your query letter will only add to the agents' overwhelming sense of ugh-not-another-Divergent-clone, and you'll rack up more rejections than you'd like to count.
Now, there is definitely something to be said about taking the pulse of the industry, which isn't the same as jumping on the latest bandwagon. Paying attention to what's selling consistently, but isn't, perhaps, at the peak of the mountain, will give you a solid sense of where your work fits and if it has a chance of flying. This happens in more general terms, like "MG is enjoying a resurgence lately" or "Regency Romance seems to remain stable in an overall volatile marketplace". Far better to understand what's out there and what sells than to throw your I'm-not-sure-what-I've-got-here project blindly into the abyss.
But. Throwing yourself into the latest frenzy is only going to frustrate you. So make sure you understand the difference.
Here comes the disclaimer: At the end of it all, still write what you love. While you're honing your craft...while you're learning the publishing ropes...while you're discovering your own, unique voice...write what you love. Even if it is an on-the-rise trend. Even if there are way too many wannabes clogging the pipes of your chosen genre.
Writing what you love isn't the same as chasing a trend on purpose. If your heart is content, then pour yourself into your work and bide your time.
I know this all too well. I write science fiction, and my favorite sub-genre is dystopian. I know, right? Because it's to the point where editors back away while waving garlic cloves and muttering curses. I write straight science fiction, too, which has become an almost-as-tricky sea to navigate. Recently, I received a rejection-that-wasn't-a-rejection on my YA SF. It was glowing praise for my story from an editor who wanted to acquire it--but encountered an in-house impasse. What can you do? Nothing, really, but press on.
I printed out the Words of Praise and hung them on my bulletin board. They read like something from a starred review, and I glance at them frequently to remind myself that, yes, I can write, and someone out there really, really liked it.
All it takes is one yes.
So don't try to be something you're not. Don't try to write something just because it's the next hot potato, and you want to get ahead of the curve. Chances are, you won't. Write what you love, and work your tail off to become the best you can be.
The reward for your hard work will come in its own time.