The most obvious and recent wave has been the wave of the vampires, which began shortly after the beginning of the twenty first century. Before the vampires were werewolves (see: Big Wolf On Campus, etc) which began its decline a little earlier on. They made their last notable stand in the Underworld series (and be honest, they were pretty badass in there) before starting to fade into the shadow of what can be known as the age of Vampires.
During this peak, it can be noted that the fantasy creature begins to change and evolve, becoming more and more extravagant. This is what is known as the 'peak' or 'high point' of the wave. The vampires are given new powers to work with the series that they find themselves in. This can range from the original concept of dracula climbing down the walls and changing into bats to much more... interesting things, such as the concept of being un-freaking-stoppable.
so to go from being an old, pointy-eared thing like this

to being the absolute invincibile, dog/bat/eyeball thing that is Dracula in Anime today...

It's pretty impressive. [/godmod]
There is no denying that these last few years were the years of vampires. From Underworld to Hellsing to Trinity Blood (I shall warn you now that I am naming ones before the beginning of the decline), the concepts of vampires were remarkable. I couldn't get enough of it. I dont' think anyone could. However, just like belly tops and hip-huggers, all trends begin to die off. During this decline, several notable series were released. Two of the most notable being Twilight and Vampire Knight. I shall not go into a huge rant about my hate of the series, nor shall I say that they are bad. They were... good in their own ways... just not my particular cup of tea. This, however, is when the fate of the vampires was put into the hands of people who took the concept of vampires too far... taking them until they were not really vampire anymore. They did not even show similar attributes to their ancestors. By now they were simply blood-sucking elves. (oh you know it's true). This is known as the decline of the vampire fad. While there are still avid fans, the true vampire has been long lost, and just like every other fad, shall fade into the past, only to be recovered again when a witty young author decides 'Hey, let's make a story about vampires.' For now, when one thinks about vampires, it's kind of a 'been there, done that' feeling. At the same time of the decline, another genre poked its head into the mainstream.
Zombies.
Oh, now don't deny it. In the past five or so years, how many shooter games have you played? In how many of them were there zombies that you had to shoot? Yes, i count the Flood as a zombie.
Halo, Nazi Zombies, Marvel Zombies, Zombie Manga... it is becoming extremely prominent in society today.

Yes, Zombies are the current rage now, even in literaturature

And you know what? I am reading this book right now!
And now zombies are evolving from the staggering slow ones that crawled out of graves to fast-paced infected forms of once living beings. Some that run, some that catch you with your tongue, some that even EXPLODE!
Then they just get sexy

I know, i know, i have a Pyramid Head fetish. But the growing popularity of zombies and the prominent decline of the vampire fad is becoming increasingly prominent, there will most definitely be a decline. As for what comes next, one must look farther back into the cycle and figure it out. When will the zombie fad get too far? What shall replace it?
Do not fear, though, just as the movie producers of today are taking our childhood shows from the 80's and 90's and making them into movies to get some extra cash (and in most cases not raping our childhood in the process), the vampires shall return and will be better than ever, back to their former glory.
As for the decline of the zombies, Personally i'm waiting for a silly romance book.