Hmm. Interesting thought from someone on GB:
”There were lots of hints that it might be something close to the doctor. All these thernes about being a policeman, the police building doors looking like the Tardis doors, a humanoid that ages but cant die. They seem to be going in the direction of the doctor was a policeman in a past life, the memories of which have bled through which is why the tardis is a policebox. Sonething along these lines.”
And someone else wondering if Brendan was a Timeless Child somehow being drained to keep others young.