Some speculation via quotation:
THE MASTER: “There is some evil in all of us, Doctor, even you. The Valeyard is an amalgamation of the darker sides of your nature, somewhere between your twelfth and final incarnation. And I may say, you do not improve with age.” – The Ultimate Foe (1986)
THE DOCTOR: “Sorry, wasn’t it obvious? The Dream Lord was me. Psychic pollen. It’s a mind parasite. It feeds on everything dark in you, gives it a voice, turns it against you. I’m nine hundred and seven. It had a lot to go on.” – Amy’s Choice (2010)
THE GREAT INTELLIGENCE: “The Doctor lives his life in darker hues, day upon day, and he will have other names before the end. The Storm, the Beast, the Valeyard.” – The Name of the Doctor (2013)
So might Capaldi be playing the Valeyard? Capaldi doesn’t look enormously like Michael Jayston (he actually reminds me of Craig Ferguson), but he does look more like Jayston than Tennant or Smith do.
I know the big V.’s supposed to be just after 12, not just after 11, but the Hurt Doctor might throw the count off. And indeed the narrative function of the Hurt Doctor might be precisely to advance the count so that Moffat can do the Valeyard storyline before he quits as showrunner ….
If true, might this have something to do with why they didn’t go with a woman or minority for the role? If the plan all along was to do the Valeyard storyline, which has the Doctor being evil, it might ruffle feathers if they finally introduced a non-white-guy Doctor, and then immediately made him/her a bad guy.
Plus, we’ve seen the Valeyard, and he’s a white male. (I assume the Valeyard is just one incarnation.)
Though the BBC may resist making the star of their show evil. I always assumed that if they did the Valeyard storyline they’d have the Valeyard and the 13th Doctor somehow generated simultaneously.