Right, i really doubt it will be an entirely new person. I think it will be a familiar and important one that we dont know much about - jkr always gives a backstory on a main character in her books, in PS it was harry himself, in CoS it was Hagrid, in PoA it was the all the Maurauders, in GoF it was Neville and the Death Eaters trials, in OotP it was snape and his parents.
So, who is big and important and who we know little about? most of the Faculty and some of the order but most noticably
Dumbledore.
I think we'll get his story in this book as the half blood prince "champion of mudbloods and muggles everywhere" and snapes in book 7