There are other schools like Hogwarts
The new Defense Against the Dark Arts teacher has a magical eye - also, he supposedly knocks Draco Malfoy around.
We meet a new, nasty female character introduced - could this possibly be the new best friend Harry might be getting? - and her name is rumored to be Icicle.
The Dursleys may be facing finacial problems. As if we didn't know the house would fall in under - ahem - certain people's weight.
Crookshanks will probably be back in this book.
Harry will not be tempted by the Dark Arts.
The title will NOT be Harry Potter and the Quidditch World Cup, although the World Cup will be an important part of the book.
They will come out simultaneously with the UK editions.
Their OPINION (this is not a fact, just what they think) is that Ron will die.
It is rumored that in the fourth book other wizard schools will be introduced and Harry, Ron, and Hermione will meet some of them.
Harry will fall in love (or, as J.K. puts it, 'he will find his hormones'). Perhaps Cho Chang?
Ron and Hermione, along with everyone else,will also fall in love with someone. J.K. has said that everyone will fall for all the wrong people.
THIS IS THE BIGGY: There will be casualties in the fourth book (meaning, people will die.) J.K. says that of the deaths, there will be one that the reader will care about.
Rowling hinted in an interview that this person WILL NOT BE HAGRID. All she will say about Ron is that people always ask her not to kill him- She will not say that he won't die. But, doesn't it seem unlikely she would kill a child? Apparently (I have no proof of this), there is an old legend of someone named Running Weasel in The 6th Century...which can mean "Ron Weasley" in English. In the legend, the man is a brilliant stratigist, and NEVER LOSES A GAME OF CHESS. With this in mind, combined with the fact that almost every name J.K. has created comes from somewhere, it would seem she may have based Ron's name on this. At the end of the legend, the man is killed by a rat. It sounds odd, but could be true. If anyone would like to research it or already knows something about the legend, I would love to hear from you.