The Series Nine Prologue appears to suggest that the Doctor has received a summons from a very old enemy. One that if he responds to will see him "embarking on an enterprise that will end in his destruction"...
Could it be, though, that the summons is actually from himself? Or rather his darkest self?
We know the Valeyard is set to rise somewhere at the latter end of the Doctor's timeline:
"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."And the Doctor is currently in the early days of that Twelfth incarnation!
Therefore if we are to take the Trial Of A Time Lord arc as gospel, he could prove unable to avoid a meeting with the side of himself he'd rather forget. Particularly if it were to prove an elaborate trap, as the Sixth Doctor's show trial by the Time Lords proved to be.
So the question the is, could this new season be setting the scene for the coming of the Valeyard?
Of course, not so long ago when the Doctor was more comfortable wearing a bow-tie he faced a brush with a dark side of himself.
Which raises an intriguing possibility. If Peter Capaldi is not approached to portray the Valeyard in a logical extension of his exploration of the dark side, could Toby Jones be asked to take the next step up from Dream Lord to Valeyard? The two share many a similarity and it doesn't take much lateral thinking to accept a retcon of sorts - the Dream Lord could well have been a mental distillation of the lurking darkness, a sort of proto-Valeyard, a new form following his first. A next step could well be to give the mental image a physical form.
After all the ending of Amy's Choice showed us he was still lingering somewhere in the universe's most guilt-ridden subconscious. The Doctor's worst nightmare would therefore be that he was granted a tangible flesh & blood existence!
Or to take it the other way the Dream Lord could manipulate him into believing he had become the Valeyard during a dream-state - taking advantage of a vulnerable mental state. He still has no answer to the question that's dogged him since his regeneration, after all.
RUSTY: I see beauty.Hatred which he's now seen twice and been able to interact with!
DOCTOR: Yes, that's good. That is good. Hold on to that.
RUSTY: I see endless, divine perfection.
DOCTOR: Make it a part of you. Remember how you feel right now. Put it inside you and live by it.
RUSTY : I see into your soul, Doctor. I see beauty. I see divinity. I see hatred.
"There was an old Doctor from Gallifrey, who ended up throwing his life away. He let down his friends and..."A life which he himself in a roundabout sense had tried to end as prosecutor at his own trial.
INQUISITOR: I call upon the Valeyard to open the case.It turns out that no, it wasn't - though the Valeyard managed to escape despite his miscarriage of justice, hiding himself behind the guise of the Keeper of the Matrix.
VALEYARD: By order of the High Council, this is an impartial enquiry into the behaviour of the accused person, known as the Doctor, who is charged that he, on diverse occasions has been guilty of conduct unbecoming a Time Lord.
DOCTOR: Not guilty!
VALEYARD: He is also charged with, on diverse occasions, transgressing the First Law. It is my unpleasant task, Madam Inquisitor, to prove to the enquiry that the Doctor is an incorrigible meddler in the affairs of other peoples and planets.
INQUISITOR: Yes. I see, Valeyard, that it is on record that the Doctor has faced trial already for offences of this nature.
VALEYARD: That is so, my lady, and I shall contend that the High Council showed too great a leniency on that occasion.
INQUISITOR: Very well. Doctor, you've heard the charges. Do you wish to say anything before the enquiry proceeds?
DOCTOR: Only that this whole thing is a farce. I am Lord President of Gallifrey. You can't put me on trial.
INQUISITOR: Doctor, since you wilfully neglected the responsibility of your great office, you were deposed.
DOCTOR: Oh. Is that legal?
What better place to hide than the Doctor's own mind?
DOCTOR: How did you get into my TARDIS? What are you?With no blurring line between dream & reality perhaps he could finally achieve the step up from dogging the Doctor's thoughts/dreams into horrifying reality. The chance to finally become so much more than just "A speck of psychic pollen from the candle meadows of Karass don Slava."
DREAM LORD: What shall we call me? Well, if you're the Time Lord, let's call me the Dream Lord.
DOCTOR: Nice look.
DREAM LORD: This? No, I'm not convinced. Bow ties? Interesting. I'd love to be impressed, but Dream Lord. It's in the name, isn't it? Spooky. Not quite there. And yet, very much here.
And just like the Valeyard had been/could be,
"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."
Perhaps Missy/The Master could even have a vested interest in helping her old foe escape his clutches once more!
MASTER: Welcome, Doctor.The question remains, though, if the Valeyard were indeed to die would she still believe herself supreme? "With the Doctor as my enemy, I always have the advantage." Perhaps she remains afraid of him -
DOCTOR: Well, I never thought I'd welcome the sight of you.
MASTER: It will not happen again.
DOCTOR: What puzzles me is why it's happening now.
MASTER: The explanation's quite simple. I want the Valeyard eliminated, and you're the most likely candidate to achieve that.
"But the Valeyard, the distillation of all that's evil in you, untainted by virtue, a composite of your every dark thought, is a different proposition. Additionally, he's infuriated me by threatening to deny me the pleasure of personally bringing about your destruction. And so he must pay the price."The mother of all grey areas?