What about "the 8th book"?

Just wondering...

There are loads of picks about what would wewe want the 8th book to be about (if there was one). All picks orodha similar plot-lines: In between the 19 years later (the trios post-war relationship), About the trios children, About Tom Riddles childhood, About the marauders schooldays, About Snapes childhood both in and out of school, About Dumbledores childhood... And a couple of other bits and bobs. (All of which I think would be interesting to read).

However.
Nobody else ever wonders about years and years and years ago... How did Hogwarts truly come to be? How about a book about the founders of hogwarts? I think that would be quite interesting, don't you? What would your favourite book be?

Just curious really...