So dystopias are the new vampires, which were the new wizards, unless the new wizards were Greek gods. (Roman and Egyptian gods are the new Greek gods.) Mermaids would be the new angels, except that they probably hope to do better than angels, which were supposed to be the new vampires. Fairies and faeries look like they might be the next mermaids, unless witches fly faster. For the picture book set, penguins were the new pirates, bunnies are back, and trucks never left. If any of the above has a head that can spin, I'm sure it's doing so.
None of these ideas are completely new, and many of the fantastical ones work best when they reimagine established folklore. That recognizability makes it easy to find a book that appeals; if you're old enough to read a vampire book, you most likely know already what a vampire is. Once you've discovered one book you like, trends also provide easy answers to the question, "What should I read next?" Since young readers tend to love familiarity, there's no reason to mess with something that works.
But what about the kids for whom the trend doesn't work? Many kids and teens do love fantasy, and yes, its ubiquity has probably created a number of fans (the degree to which entertainment creates taste, as a certain visible article suggested recently, is a subject for another post). But customers do also ask for realistic fiction. There's plenty for first chapter readers, and for older ones, there's the parallel trend of "books like Wimpy Kid," and there are--gasp--books that have been around for a few years. Beyond that, there are definitely options, just... not so many.
I just hope good realistic fiction--the funny, the serious, the sweet, the scandalous, and everything in between--isn't being passed over just because humans aren't supposed to be the new mermaids.