My money is on dumbledore or dean thomas.
Way i see it, snapes gonna be the big guy in seven, he's gotta eventually be dark arts teacher then so his background and full story's gonna come out along with the stuff with james and lily etc.
That means dumbledores story and mysteries have all got to come out in this book, particularly since he'll prob be killed off in 7 when harry will take over as the only person voldy is afraid of. He was nearly beaten at the end of book 5. He will be the key figure, it will prob explain his background, blood ties, why he trusts snape etc. He also fits the role doesnt he, he has ensured all students esp halfbloods and muggleborns are accepted at hogwarts, he is a griffindor, an excellent wizard, a likely heir to godric griffindor, who would be a king. He could equaly be a prince who is a halfblood or a prince of the half bloods