NB: This would require the ending of âTwice Upon A Timeâ to fade out at the point of Capaldiâs regeneration, right before Whittakerâs eyes appear.
At the conclusion of âThe Time of the Doctor,â the 11th Doctor mistakenly thought that the Time Lords were granting him a completely new regeneration cycle. They werenât. They granted him *one more* life to enable him to complete his work on Trenzalore and defeat the Daleks.
During âTwice Upon A Time,â the 12th Doctor is struggling with the decision of whether to regenerate or to refuse, unaware that this was his final incarnation. Ultimately, this decision is taken out of his hands. As he regenerates, the artron energy dissipates into the heart of the TARDIS. The Doctor has passed away. The TARDIS travels on and reconstitutes the Doctorâs personality and memories to create the being known in âThe Day of the Doctorâ as âThe Curator,â an artificially generated being that is able to shape itself into different forms of the Doctorâs past faces (ie an older fourth doctor, maybe a younger first doctor, younger seventh doctor, older ninth doctor etc). It remains as a monument to the Doctorâs history and ensures that his work lives on.