It's kind of funny, too. I mean, vamps have been around in one form or another for ages now. They're seeing a resurgence thanks in part to Twilight and to other urban fantasy/paranormal romance titles.
Wikipedia's vampire page lists 18th century poetry as the first appearance of vamps in literature. We saw the rise in urban fantasy recently, and the marriage of paranormal aspects with every other existing genre of books I can think of. Of course vamps play a huge part in all of the paranormal subgenres, but so do ghosts, witches, werewolves... Everyone is looking for a being that can captivate audiences but hasn't had the exposure that the vampires have.
I like this because that means more and more people are going back to traditional mythology to breathe new live into beings that some of us maybe aren't familiar with. Nicole Peeler, listed above, has introduced Jane True, a half selkie who discovers there are many many beings living in our world. This was the first time I'd come across selkies in fiction. Super cool.
The newest trend is fallen angels. So far, I've read two books (one adult and one teen) that were about fallen angels, and one horror that included some of their mythology. I also received another teen fallen angel book. I have to admit that I am very captivated by the actual history of fallen angels, separate from the religious perspective. I don't read "Christian" fiction, but as another being whose story is foreign to me, this trend of the fallen angel as the tragic hero is one that appeals to me for sure.
Another one that hit me was the banshee. No one is writing about the banshee and it's such a cool concept for a character. And then I received Rachel Vincent's My Soul To Take, and guess what it's about -- a bean sidhe (or banshee). Vincent had already impressed me as doing something fresh and new when she introduced her Shifters series with Stray.
I think, though, what beings are big aside, it's the author's ability to capture readers that really counts. It's the author's ability to turn something that's been done into something that's completely new to the reader. It's not the next big character so much as it's the next big author and how they present their ideas. I've read plenty of vampire books in the past year that struck me as being totally different from everything out there. I've read others that are incredibly similar to others that fellow readers love and adore and I'm not so impressed with. So there's kind of a market for anything right now.
It's a matter of taste and it's a matter of talent. Regardless of what shape a character takes,demon with a soul, vampire who refuses to eat humans, vampire who seduces humans, werewolf, angel, fae, whatever, it's how they're presented that really makes all of the difference.
The market may be inundated with the undead right now, but I don't think vampires will ever truly go out of style. Their popularity may ebb and flow, but if the trend is moving away from them now, they'll come back in a few years and be fresh and new again. That's kind of how everything works in pop culture. The fact that we're seeing more and more beings introduced, though, means that authors are mindful of what readers want. Authors are looking for something new to please their readers. They're not content just to throw a bloodsucker into a story to make it appeal to the current market, they're thoughtfully seeking out something new for their fans.
So I love the trend of new beings in books, and I also love that so many new and talented authors are bursting onto the scene these days. Take advantage of it readers!