Doctor Who - Series 8

Finally finished up Series 7 a few days ago. Spilling out my headcanon for who the mysterious person at the end of the final episode is:

 Spoiler

They never revealed how the 8th Doctor died nor did they ever showed his regeneration into the 9th Doctor. As far as I'm aware, it's actually the 8th Doctor but he's aged immensely because he spent a lot of time traveling alone after what he did in the Time War. We all know the 9th Doctor and beyond couldn't forget the Time War and felt great guilt. So in a way, the 8th Doctor probably did something that shamed him immensely. No regeneration or death shown? It leaves room for the 8th Doctor to be this guilt ridden old man. The 11th Doctor can't know the actions of his future regenerations for it can/will cause a Time Paradox. It can't be a future Doctor, and obviously if John Hurt were to the 13th Doctor he's need to live out at least another 7-8 years of his life when he's already in his late 70s if he's going to be the 13th Doctor, and that's going to be extremely risky.

We can also attribute this to be a regenerated body part (similar to the Meta-Crisis 10th Doctor), but that's less likely. Even less likely is this is another Remnant of the Doctor made real.