Publishing is fraught with trends. This is for two reasons: one, would-be authors (who are also readers) see successful books and want to write similar books in the hope that they will also be successful, and publishers (who always want The Next Big Thing™) see successful books (either theirs or their competitors') and want to publish similar books in the hope that they will also be successful. (It's a vicious cycle.) Unfortunately, only a small percentage of these books catch on and become bestsellers, the rest fail to earn out their advances, authors and publishers everywhere are confused, and the process begins again when a new trend starts to catch on.
I generally advise you, gentle readers, not to play the trend game because 1.) the trend will likely be exhausted by the time your book comes out, and 2.) even if it isn't, there's no guarantee your book will be one of The Chosen Ones that readers everywhere simply MUST HAVE. Which trends in particular do I think are nearing their ends? Well, I'm glad you asked.
· The [Your Name Here]'s Daughter/The [Adjective] Wife. Archetype: Audrey Niffenegger's The Time Traveler's Wife. I recently mentioned my personal grievances with this trend to a colleague, who simply responded, "Yeah, but The [Your Name Here]'s Wife sells!" (The trend, not any title in particular.) Which is true! At least, it's true for now. As the market becomes saturated with this particular variety of women's fiction, however, I think more and more consumers will turn elsewhere for new reads.
· Vampires/Werewolves. Archetype: Stephenie Meyer's Twilight series. No, really. Authors who have already established themselves as brands with this subject, like Meyer or Charlaine Harris, can pretty much continue to write vampire novels until the turn of the century. You, who are just starting out, do not have this luxury. Vampires and werewolves will always be cool, but they won't be as cool as they are now for a long time. Putting werewolves in space or making vampires fallen angels will not increase their coolness. Trust me.
· Mash-ups. Archetype: Seth Grahame-Smith's Pride and Prejudice and Zombies. More or less the exclusive province of Quirk Books, this relatively short-lived trend probably won't survive long after the publisher's release of Android Karenina later this month.
· Anything based around the world ending in 2012. Archetype: Too many to count. As you might expect, this trend will cease to be cool on or around January 1st, 2013.
While I don't want to tell you what to write, you might notice that the women's fiction and YA crowds are well-represented here. This means (as usual) two things: first, they're bigger markets, so you're more likely to sell a greater number of copies if your book succeeds, and second, they're bigger markets, so there's a lot more competition. O, the cruel double-edged sword of publishing!
Incidentally, it also might mean we need a few more non-YA bro-oriented books out there. Just saying.