He is never going to do the origin for the same reason he is never going to do the time war nor neither reveal the Doctor's name - it is always going to be a disappointment.
We shall see. However, in nearly all situations Moffat has to build up to something in order for it to be a disappointment. No one can have their hopes dashed if there's no hope to begin with.
I'm now bothered by Clara's line that she saw 11 faces, all of them the Doctor. If the Doctor lived what many fans believe to be a full Time Lord life, there would be at least 13 faces and Clara would have likely crossed paths with them all.
In any case, it seems to indicate that we're looking at a past incarnation.
Anyway, I personally appreciate Moffat's crack at the legend of Doctor Who and am rarely disappointed (although I can certainly wish that some elements were better realized than others).