As far as trending goes, speaking from the standpoint of someone that buys an absurd amount of books, I've never much cared about whats popular as far as picking what to take home from the book store. After all I have to like it, and it doesnt matter much to me what others like.
I do agree at lot with what Wordmaker said about say vampires -or any other over used product- because I tend to get annoyed by reading the same ole thing again and again.
A good example, the zombie genre, after World War Z and the Walking Dead took off, the selves where flooded with zombie books, but very few offered anything new, and the authors doing their take on World War Z, well I had already read that, and didnt need to read a knock of version.
I will say some authors manage to take concepts that are way overused and put their own spin on it and make it fresh. A good example would be David Wellignton, I think his vampire novels are nice and different, yes he uses vampires but they are the more 30 Days of Night style vamp, and no one sparkels, so im ok with it.
Also his Monster Island books have a different feel to them than other zombie type books.
Just my two cents, make your stuff have its own feel to it, make it good, and don't worry about whats popular or when it will be published. People are still buying the WOT, LotR, and Dune books after how many years?
Trends come and go, good books are always good books