The missing chapter

[pandrea100]

No: HPFGUIDX 103689

On her website JK talks about a chapter that was originally supposed 
to be in Book One, then she tried it in other books, but it never fit 
right but now she is firmly placing it in Half Blood Prince.  So 
what's this chapter about?  Obviously not something directly related 
to the plot of any one book.

My guess is it is about the founding of Hogwarts.  I can imagine the 
first book giving us the background to how the four Founders created 
the school, which we are about to be introduced to when Harry goes 
there.  But it probably held up the story even more than the 
introduction to the whole world - which she said was hard to write, 
to get all the information in about how magic works, how Harry 
defeated Voldemort etc.

Then it could have fitted in with other books, for instance in CoS 
explaining more about Slytherin.

If the Half Blood Prince turns out to be Godric Gryffindor, then a 
chapter about the founding of Hogwarts would fit in perfectly.

Any other suggestions for what this missing chapter is about?