I figured it was the initial Ninth Doctor a few days back, but something bothers me. As a side note, seeing as the Doctor despises that form of himself for revealing his name, I'm thinking that the Doctor's name is also essentially the key to the Time Lock. If ever uttered by the wrong person in the right place, I imagine it could unleash the Time War once more, and cause the use of the Ultimate Sanction to finally occur. What I wonder is this: the Moment was supposedly used by the Eighth Doctor. Assuming this is entirely canon, that would mean that the Doctor that fought all through the Time War would have been the Eighth Doctor. With that in mind, John Hurt could actually be the Eighth Doctor, heavily aged from a very long involvement in a war removed from time, regenerating after his use of the Moment to seal the Time Lock in order to seal away the man that did what the man known as the Doctor never could have done. The jacket of the Ninth being worn over the clothes of the Eighth is easily explained as the Doctor simply transitioning throughout the Time War, with the Ninth keeping the jacket potentially as a reminder of his great crime. A Time Lord can force a regeneration, which is seen in classic Who with Romana I and Romana II, as well as when the Doctor is punished on Gallifrey and forced to regenerate by the council as part of his punishment (which, if I remember, was the cause of him transitioning from the Second Doctor to the Third Doctor), meaning that the idea of a forced regeneration would be entirely canon. It would also finally give us an explanation as to how and why the Eighth Doctor turned into the Ninth. It seems to be a subject that we have missed greatly and that nothing that's considered canon has touched on.
Now, how about Clara not seeing that part of the Doctor's life? Easy. As the Time War is time-locked, so is that part of his life. No amount of gallivanting through the Doctor's time steam will ever allow a person to return to that war without it being reinstated back into the normal flow of time. That would mean that there's a part of his life that absolutely nobody in existence truly knows about besides himself, and it also happens to be the part of his life that he's most ashamed of as a man that took on the title of protector.
With that in mind, my idea for why the Doctor's name is the key is simple: why else would the Doctor have to try so desperately to never reveal it? Why is it so imperative that the wrong person never get their hands on his name? Why would it anger him so much that he ever used his name for any purpose? It must hold great power, of course, but it can't just be that. There must be something behind the name of the last Time Lord that requires special consideration to keep his name free of being common knowledge. Speaking his name under the correct circumstances must have the ability to bring about great disaster.
And thus, I feel the 50th Anniversary Special will give us closure on the Eighth Doctor once and for all. I think it's the best they could do with Christopher Eccleston not coming back to the show for the 50th. Regardless, if they go the route of the Eighth and with him regenerating at the conclusion of the Time War into the Ninth, well, it would simply work. He wouldn't have necessarily been born in battle, but he would have been born at the conclusion of the Doctor's darkest moment. It would give a nice explanation as to why the Ninth Doctor was seemingly much darker than the Tenth and Eleventh, and give some credence to the Doctor reverting to great darkness at certain moments (such as at the conclusion of his first meeting with Donna as the Tenth).
