~ in book 2, we learn about polyjuice potion which is used later in the book for Harry, Ron, and Hermione, but even more importantly, by Barty Crouch, Jr. in book 4 to impersonate Mad-Eye Moody
~ in book 1, Harry gets his father's invisibility cloak, which he uses on many different occasions in all the books, and it is Barty Crouch, Jr. (again!) who uses it to 'hide' at the World Quidditch Cup
~ we are first introduced to house-elves in book 2, only for them to come back in a big way (and an important way) in book 4
~ I'm sure you can name many more instances!
My point is, until something is 'introduced' to us, it isn't really a factor in the stories . . . the polyjuice potion was irrelevant to us in book 1 and had not yet been introduced. My point is this, most especially with book 4, which has the most recent of 'new' things we've learned, what did we learn in book 4 (possibly about the forbidden curses, having 'moles' in the school, etc.) that may be more significant in book 5? It seems J.K. really enjoys mentioning things that seem run-of-the-mill for a wizarding school (or even downplays their significance) only for them to turn up later as a much bigger deal! Until book 4, I'm not sure I would have suspected that a teacher wasn't who they really were (nor did any of the teachers or students at the school!), but now that I've been reminded that polyjuice potion isn't just something the trio learned about--adult wizards use it--I'll be much more suspicious of teachers from now on!
My speculation, though I look forward to hearing yours and seeing if I get reminded of any other theories

Wow, sorry so long! I've just wondered how many little clues J.K. has given us to foreshadow her newest book that I've probably just read over or not thought about in terms of something we know about coming back. Looking forward to hearing your thoughts!!
