If you're currently penning a fantasy novel centering on a pious zombie vampire from an alternate 19th century in which science/alchemy/the Bible has predicted the earth's impending doom, uh, please send it to me, because that could be awesome. More importantly, though, stop doing it.
Everyone wants to be The Next Big Thing™, and no one's going to fault you for that. It's important to remember, however, that what's big now isn't necessarily what's going to be big in one to five years, which is probably the earliest any book you're currently writing/shopping to agents/editors is going to be published. I don't know about you, but I don't think vampires are still going to be cool in five years (at least, not as cool as they are now). Zombies will probably suffer a similar fate.
If you've got representation now and your novel seems to be along the same lines as other novels that are just starting to become big, that's great news, and you'll probably luck out and catch part of the wave of whatever craze is coming next. If not, then you may not. Luck is a big factor, friends. Talent and hard work are necessary, but can only get you so far.
The moral of today's post: write the best book you can, do everything you can for it if/when it's acquired, and don't worry about what the other kids are writing. Just like with stocks, trying to time the market is probably only going to get you burned.
Next week: Monday Mailbag, featuring responses to your questions!