Davies later revealed the cut passage, which said that the Doctor and Donna had gone “Beyond the Realm of the Boltzmann Brains”. Boltzmann Brains are cool. Basically, the idea is that given the infinite size of the universe and the vast chaos it contains, it might actually be more likely that a brain would randomly materialise out of nothing with the memories of an entire life behind it, than it is for intelligent life to emerge over billions of years through the processes science currently tells us happened.
So what if that’s why Davies cut the line out? Because it gave away the true origin of the Doctor. Wouldn’t that be fitting? For the person whose past has always been their biggest mystery to turn out to have no past at all? It would be like the Rey twist in The Last Jedi, but times infinity.
The Doctor is a Skywalker
We’re on Disney+ now, why the hell not?
The Doctor is Their Own Future Self
This one would admittedly be more likely if Steven Moffat was still in the big chair, but Russell T Davies’ is not above some timey wimey when the mood takes him. We know a surprisingly large amount about the Doctor’s future. We know, for instance, that at some point they retire from TARDISing for a bit to put Tom Baker’s face back on and become the curator of the undergallery. We even know that, although it was never confirmed on screen, at some point they look like Colin Salmon and download their brain into The Library to keep River Song company.
But what, really, is the ultimate End of the Doctor? What else, when all is said and done, could it be? What if, rather than “revisiting a few old favourites”, the Doctor ultimately revisits all of their old faces? What if, like the Master during the Time War, the Doctor wipes their own memory, regenerates into a small child and falls through a dimensional rift, dropping themselves off right at the beginning of their own story and closing the loop?
The Doctor is a God
The multiverse of Doctor Who is positively littered with celestial beings. The Toymaker, the Beast, Fenric, the White and Black Guardians, that frog from another universe in “It Takes You Away”. Pin their photos to a dart board, put on a blindfold, throw a dart. Wherever it lands, that’s your back-story, right there.
