Other Doctors will show up.
Sixie is obviously going to show up, what with him being the main character and all, and Seven will obviously appear at the end and possibly be given a line of dialogue or two (to convey the change through the audio format), taking some slight liberties with the originally broadcast sequence. But the presence of the Valeyard as the seeming main villain indicates that the Doctor's future is going to be important to the arc somehow, as he is an amalgamation of the darkest sides of the Doctor taken somewhere between his Twelfth and Final incarnations, as per The Ultimate Foe. Seven will likely cause Sixie's regeneration into him somehow, being the Manipulative Bastard that he is, but such a plan could possibly create a massive temporal nexus point causing a localized collapse of the Doctor's timestream, drawing other Doctors to the region as they attempt to figure out what's up. Ultimately, the strain of his timestream being so heavily distorted as he battles his possible future self could weaken him badly, making him weak enough for the Rani's attack to push him over the edge.- Disregarded in the end, although technically the final episode does feature 'two' Doctors, as a version of the Sixth Doctor from a future created by the Valeyard changes his own history to 'kill' himself so that he will thwart the Valeyard's plan by regenerating).
- There are a few glimpses of the Seventh Doctor before and during the Sixth Doctor's regeneration, which technically counts as a surprise appearance.
Some of the Sixth's last words were purely mental
- The circumstances of the Sixth Doctor's regeneration were also depicted in the Past Doctor Adventures novel "Spiral Scratch", which showed the Doctor regenerating after suffering a chronal energy drain to stop a pan-dimensional parasite. It is possible that the regenerations depicted in "Scratch" and "Brink" could be different perspective of the same event, since most of the Doctor's last words in "Brink" were said when Mel was apparently unconscious so he's just thinking about his imminent death rather than outright talking to himself. It is not impossible that the Doctor could have recovered from the chronal energy drain he suffered in "Scratch" on his own, but the damage he suffered from exposure to the Rani's radiation finished the job.
