I think that I've mentioned a number of times that when we've met face to face, I always encourage students to image what they would write in the genre that we are reading any particular week. We've done this a number of times in class to greater and lesser success, but I want to try again.
What story would you tell for a YA audience? No particular word count. Have fun with whatever ideas you have. You can write in any genre, but it's for a YA audience.