A most interesting scene in The Magician's Apprentice was when Colony Sarff (a seriously great creation) visited the planet Karn in his/its/their search for The Doctor, there he meets the Sisterhood of Karn who tell Sarff to leave and deliver the message to them. It is suggested that unknown to Sarff, the Doctor was listening. Planet Karn, the Sisterhood of Karn? You need to have watched the mini episode entitled The Night of the Doctor that preceded The two parter The Day of the Doctor. This mini episode explains the regeneration known as The War Doctor, if you haven't seen it then you should have...
Now does Planet Karn make sense? And nice to see the Ninth Doctor again.
So if the Doctor can, at least if he regenerates on Karn, have some choice as to his next regeneration's attrivutes and appearance does that go someway to explaining why the Twelth Doctor bears a surprising resemblance to Caecilius from The Fires of Pompeii? But the Eleventh Doctor regenerated into the Twelth Doctor in the TARDIS.
As an aside, note that the Sisterhood of Karn offer The Doctor the choice of "man or woman", does that mean that The Master regenerated on Karn or was his regeneration into Missy random? Either way the possibility of their being a female Doctor seems higher, that would be a mistake I feel but then I'm not in charge of Doctor Who.