In "Trial of a Time Lord: The Ultimate Foe", the Master tells the Doctor: "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." Because of the War Doctor, the "twelfth incarnation" of the Doctor would be #11 (Matt Smith). So it can't be the Eleventh Doctor himself, but a Doctor after him. However, in the novelization, the Master says that the Valeyard is an incarnation of the Doctor "Somewhere between [his] twelfth and thirteenth regeneration." The twelfth regeneration was from #10 (David Tennant) to #11 (due to using up a regeneration to remain the same, resulting in the Metacrisis Doctor), and the thirteenth regeneration was from #11 to #12 (Peter Capaldi). So it would seem only Matt Smith's Doctor is between his twelfth and thirteenth regenerations, but as you remember, it can't be him. However, there is someone who fits the bill: the Ganger Doctor from the episodes "The Rebel Flesh" and "The Almost People". He's between Matt Smith's incarnation and the Doctor's final incarnation. He's also between the Doctors 12th and 13th regenerations. Only issue is how he survived, since it appears that he died in "The Almost People". Perhaps, because he's a ganger of a Time Lord, he was able to somehow regenerate after being turned into liquid, but ended up changed after his liquid mixed with the monster ganger. (I kinda wanted the Metacrisis Doctor to be the Valeyard, especially after seeing David Tennant as Killgrave in Jessica Jones, but the math doesn't work, so I guess it wasn't meant to be.)
Let not forget about the Dreamlord who I assume is pretty much the Valeyard. My theory is that 10th Doctor was really vain and did not want to regenerate, that vanity and greed created an evil entity when 10 regenerated into 11. That entity was in a non corporal form (Dreamlord) then some how found a way to manifest in a physical form (the Valeyard). Even though the regeneration number is slightly off, I do like the idea if the Metacrisis Doctor being the Valeyard also. This is the only known physical manifestation offshoot of the Doctor. Also, since he has a Timelord brain but only has one heart he could have reason to go after the 6th Doctor's remaining regeneration's and become a whole Timelord again. We already know how vain he was, and he had a streak of evil in him (Family of Blood, Runaway Bride, etc..) and was willing to commit genocide with the Daleks and Davros. Perhaps Rose died, and he went crazy and escaped the parallel universe. Who knows!
This isn't a theory so much as just a fankwank idea I came up with that I'd potentially go with if I was running the show, maybe the Valeyard could be from an alternate timeline, and was what the Doctor became, but the Time War rippled time and changed the course of the Doctors own history, thus neglecting the Valeyard's existence and necessitating him to steal an earlier incarnations body to cement his own existence. I also quite like the idea of the Valeyard being a parasitic entity created from the Doctor, that needs his body to cement his own existence again. Just my fanwanky ideas as I say. Another explanation could be the Master was trolling the Doctor epic style, figuring out the Doctor has surpassed his initial 12 regenerations and hence his statement about the Valeyard being "between your twelfth and final incarnation" was accurate, but extremely vague since it could be anything from the 11th Doctor onwards.
I prefer to think of the Valeyard as some made up ploy by some Timelord that backfired and so will never be brought up again in the show.
Actually, I never thought about it before, but the Time War may have either erased the Valeyard's existence or caused him to manifest at a different point in the Doctor's life. After all, the Doctor's death on Trenzalore was undone, and that didn't even involve a full-blown Time War. So it's entirely possible that the Valeyard was unmade or his origin was altered. So, yeah, let's bring back David Tennant as the Valeyard! You mean you don't want an episode where the Rani returns and creates the Valeyard with the help of Professor Lazarus, the Nestene Conciousness and Abzorbaloff?
Well, to start with, it has to do with the fact that the Doctor is half human, on his mother's side...
The simplest explanation is this: With The Valeyard being set between the second last (Tennant) and last regenerations (Smith) -- remember Capaldi is the First Doctor is the second set -- logic would follow that The Valeyard is the Meta-Crisis David Tennant Doctor, trapped on Pete's World with Rose. He is the only Doctor that fits the *between* theory, and since Tennant has shown willingness to reappear on Who, perhaps this was planned from the beginning. Maybe Rose dies, he loses his mind and wants revenge, and hey presto Valeyard.
While the War Doctor fits better into continuity than a retcon of that magnitude has any right to (and would've been perfect if they hadn't started saying the numbers on-screen in the Moffat era), it was a concept that only came about while planning the 50th anniversary special. Still, Valeyard-as-Metacrisis-Doctor is a compelling theory I am not opposed to. Heck, I liked it when the comics played with it pretty much immediately after "Journey's End" aired.
Other than in the 1996 movie, when was that ever mentioned? (I don't accept that as a "fact", btw) "Saying the numbers on-screen"? Huh? There was an acknowledgement of the fact that there were three Doctors in the 10th anniversary story called "The Three Doctors".
I think they retconned that in a novel shortly after the FOX TV movie came out. Then, when NuWho can along, they basically said he was entirely alien in the pilot. Neither do I. I was continuing a joke about bringing the Valeyard back in the worst way possible.
I don't know, there's potentially a really good Valeyard origin story, especially if David Tennant comes back to reprise his role as the Metacrisis Doctor gone bad. Maybe if Neil Gaiman and Steven Moffat co-wrote the episode, perhaps...
Tbh why get David Tennant when you can get Michael Jayston One nitpik though, how does a Metacrisis Doctor/Valeyard go from looking like David Tennant to Michael Jayston? A regeneration would be necessary but with the metacrisis Doctor being half-human, that pretty much making stealing the Doctors regenerations being necessary, how does The Metayard change appearance initially?
Just say that the Time War changed the course of events, so that the Doctor never took that form, but that the emergence of the Valeyard was inevitable because he's fundamentally a part of the Doctor's psyche.
Metacrisis could've pulled a Master/Keeper Of Traken move, taken someone else's body to keep going. Of course, that would preclude us getting Tennant in the role, which would be a crime.