At some point in their life, every author has asked this question:
"What should I write?"
I hope you weren't expecting some long complicated answer, because you won't get one. The only answer is write what you love.
Think about books for a second. Think of your favorite books. What kind of books make you stay up late after bedtime, thinking just one more chapter...one more chapter...and it turns into three hours? For me, it depends, but in this case I'll say epic fantasies. I love the adventure, magic, and just plain awesomeness of Lord of the Rings-style sagas. I like dragons and wizards and cool stuff like that.
Is that what I'm writing? Pretty much! Well, it's not on the same all-out war level of fantasy as LOTR. But it has dragons and...you get it.
What you should never, ever do is write on a trend. Dystopias are popular now, sure, but publishing is sloooow, and by the time you're even done writing the trend will be long past. It's already starting to die, and that's a problem for your super-trendy book. I wouldn't have wanted to be the author of Twilight's agent right after the book's huge success. Just imagine how many people sent her their vampire romance novels just because Twilight did well! Never write something just because you think it will sell well. Fantasy novels aren't the trend right now, but I'm writing one just because I love it that much.
Here's a good rule of thumb: If you were a reader and saw your book at the library (or bookstore, for the germophobics), would you pick it up? Would you like your own story? If no, chances are you aren't writing what you should be writing. Do you, as the author, enjoy your own story? If not, your readers will pick up on that, and they won't enjoy it either.
So, here's my say on what you should write: Write what you love. Write the story that wants to be told. Write the words that keep you up at night, distract you during class, and run through your head all the way home. Write a story you truly enjoy. Write the book that keeps coming back, long after you've told yourself to forget about it.
Because anyway, it's your story, so nobody else can tell it better than you!