Following on from a multi-Master story, the Christmas episode will indeed be a multi-Doctor Story; featuring none other than the First Doctor, played by David Bradley. After being mortally wounded by a Mondasian Cyberman, The Doctor Falls ended with the Twelfth Doctor refusing to regenerate, arguing “I will not change!” before hearing those same words repeated back to him. The First Doctor then comes into frame telling the Twelfth Doctor “You may be a doctor, but I am the Doctor. The original, you might say.”
It is too early to speculate, but it looks like the 2017 Christmas episode will feature First and Twelfth Doctor helping each other come to terms with changing into a new body. David Bradley will feature as the First doctor, but not for the first time as he has previously played the First Doctor in the 50th Anniversary Biopic, An Adventure in Space and time. He has also previously appeared in Doctor Who, playing the merciless Solomon in the Series Seven episode, Dinosaurs on a Spaceship.
It’s unsure what the First Doctor is doing here, but it looks like this may take place during The Tenth Planet story from 1966, which also happens to be the story in which the Doctor regenerates for the first time. The Christmas Episode will be Peter Capaldi’s swan song before we are introduced to a new Doctor.