So, as I've been doing a few things in the kitchen, I've been letting the final scenes of the Series 7 finale play over in my head, and I wonder which incarnation of the Doctor that John Hurt is supposed to be. I shall share my theories.
1.) John Hurt is the final incarnation of the Doctor, and as a result of his entire life of conflict and combat, he's turned slightly dark, turned away from what it meant to be The Doctor. But, if that's the case, how would the 11th Doctor know about him and why would he be the Doctor's greatest secret.
2.) John Hurt is supposed to be The Valeyard, or some version of it.
3.) The incarnation that we know as the 9th Doctor wasn't really the 9th, but possibly the 10th. John Hurt is the 9th incarnation, the one that fought in the Last Great Time War, and was injured badly enough at the end of the war, that he regenerated into Eccleston prior to the series revival.
4.) John Hurt is actually a younger version of the very first incarnation of the Doctor, prior to his stealing the TARDIS from Gallifrey, and in his youngest days, he had done something terrible, so terrible that he dared never to let anyone know.
Just my thoughts mind you.