I hereby predict (sound pompous enough? ;) that book 7 will end, or Voldemort will end, anyway, in the "always locked" room in the Department of Mysteries.
I also believe that the archway that killed Sirius and those flying brains will be important in the final battle scenes and/or whatever final scenes end Voldemort.
Having studied JKR's style a bit too obsessively lately, I see that she leaves hints for the future in certain ways. Rereading book 5 I am a bit stunned at how often Montague (whose sojourn in the broken vanishing cabinet gave Draco the crucial information he needed to get Death Eaters into Hogwarts at the end of Book 6) was mentioned. She just casually throws Montague in several times, just to remind us of him. His angry parents show up to talk to Umbridge, he's in the hospital wing getting medicine from Madame Pomfrey, etc. I didn't notice that the first time I read the book, or the second, either. I had to read book 6 and THEN reread book 5 to notice.
With that sort of writing style in mind, I see the experiences that Ron, Hermione, Ginny, Neville, Luna and Harry had in the Department of Mysteries in a new light.
I was really struck by the way only some of the students were drawn to the arch that later killed Sirius. Harry was fascinated by it, but Hermione knew it was dangerous, and explicitly labeled it as such.
The flying brains are also mentioned several times. Ron tries to play with them when his brain is addled from an unknown spell. Then the brains apparently come close to killing Ron.
The door that melted the magical knife is probably the "always locked" door that has behind it the power of love. Dumbledore tells Harry about this on pages 843 and 844.
These things were all emphasized to no known purpose in book 5 (or book 6, for that matter), and so I feel they are very likely to be very important in the last remaining book.
Prediction made. Now I have to take care of a crying 1 year old. ;)