Well, there is a definite precedent for 'in between' regnerations. The Watcher was a figure in between 4 and 5 and ease the transition. The Valeyard was established as being between 12 and 13. And it's fairly clear that Hurt is in between 8 and 9. 4, 8, 12; there is a definite logical continuity between when the in betweens happen. Also because the Valeyard has been clearly established as between 12 and 13, it doesn't make since for either Capaldi or Hurt to be them. I suppose the in between 8 and 9 thing could be a red herring, but I doubt it. Hurt's costume is clearly not quite Eccleston, not quite McGann.
I think the most likely scenario is that the Valeyard is going to be a big bad for Capaldi, a future to try and change. And when he can't change it, to prepare time and space for.
I think the most likely scenario is that the Valeyard is going to be a big bad for Capaldi, a future to try and change. And when he can't change it, to prepare time and space for.