As the original post says, it'll just be "outtakes," so it'll be whatever her editors managed to convince her to remove from the previous books. Judging from the size of her recent books, book eight is going to be thin.
I wouldn't be surprised if she eventually licenses the Hogwarts world to someone though - there's a goldmine in spinoff books just waiting to happen. (Which isn't to say that the main series hasn't been a goldmine in it's own right.)