Tempting as that might seem, fads – like fashions - invariably change. Remember not so long ago boy wizards were all the rage! Unless you are a genuine author of horror, forget being a slave to the market. Leave that to the trendsetters and the dedicated genre writers. An author should always strive to write from the heart, to write what is nearest and dearest to him or her. Cliché as it sounds, be true to yourself. You’ll be a happier person and a better (if probably poorer) writer for not selling out in the hopes of making an easy buck.
On the other hand, if by say adding a vampire genuinely enhances your storytelling and legitimately drives the plot, by all means let a fanger join your cast of characters. AS LONG AS IT IS RELEVANT TO THE TALE. Take my upcoming third book in my Terrath series as an example. I won’t give out any spoilers, but in Magic’s Resolve I purposely introduce a supernatural element to one of my Goblin characters. In no way am I trying to follow trends. This is merely the natural progression of that particular character’s planned development. While it happily coincides with what is hip at the moment, I wasn’t influenced by anything other than evolving and enhancing the storyline.
But I guess in the end it comes down to the individual. Only you can decide if you’re a sheep that runs with the rest of the flock or bravely follows your own path.