Ok everything im about to say is fact based on author interviews and the timeline through the other books. In book 7 Harry will be turning 17 years old, Bill and Fleurs wedding will take place somewhere (probably in the beginning, i think something might interupt this wedding), important things revealed concerning Harry's Aunt Petunia (will discover what Dumbledore meant when he said "Rememeber my last" in his howler to her and Jk said there is more to this character then meets the eye, she isnt a squib, and she wont preform any magic), Umbridge will be in this book, Rita skeeter will be in book 7, the two way mirror from the 5th book is likely to show up again, Krum will make an apperance, Harry's eyes have some importantance in book 7, Wormtail's debt to Harry will show up, more info given on James and Lily Potter, one of Harry's classmates becomes a Hogwart's teacher, significance of Godric's Hollow (where Harry's parents lived) and Godric Gryffindor (you put two and two together), and the book is likely to be released mid 2007.
Whew. I know i didnt really speculate but just thought people might like to know some of the real facts to help them down their own paths of speculation.