Doctor Who is a show that due to it’s on-going serialized nature, low-budget, production issues and being continued well beyond what the original creators could have ever imagined, it naturally leaves many plot holes and unanswered questions due to the writers not believing that they’ll be addressed again, or because they just forgot about them
One of the biggest being ‘why hasn’t the Valeyard shown up yet in the new series?’, and there have been many fan speculated theories on that, the most logical being how the Doctor’s own timeline has been altered so much that he never existed, to him being a version of the Great Intelligence when he goes to stop all of the doctor’s victories in ‘The Name of the Doctor’, but my personal theory, while not watertight, is a lot of fun to think about
Just a quick refresh for those who don’t remember, or just don’t know at all, the Valeyard appeared in the sixth doctor story arc ‘The Trial of a Time Lord’, which heralded the end for Colin Baker’s run as the Doctor (though that’s mostly due to the following series being cancelled) and Nicola Bryant as the assistant Peri Brown, and it also introduced Bonnie Langford as Mel (regrettably), and focused on the Doctor standing trial in front of the Time Lord counsel about his behavior, with the Valeyard acting as the prosecution. The Valeyard was a dark reflection of the Doctor who was, according to the judge of the Doctor’s trial ‘created when the doctor was trying to experiment with regeneration, sometime between his 12th and final incarnation’, who acted as the prosecution on the Doctor’s trial so he could claim the remainder of his regenerations, since he was unable to do so himself.
Now, let us not forget that the definition of what the doctor’s ‘12th and final incarnations’ are is a bit of a tricky subject; especially since his regeneration cycle has been extended by the Time Lords. During the Time War, the Doctor didn’t call himself by that name, since he felt he was betraying what that name stood for, and so all future and past versions of the Doctor dis-owned that version of him, meaning that every version of the doctor after the time war, is actually an incarnation older than they say they are (so the 9th is actually the 10th, the 10th is actually the 11th and so on and so on), plus, during the episode ‘Journey’s End’ the 10th Doctor (or the 11th, if you will) did regenerate, but kept the same face (due to vanity issues, according to his future self), so that means that the version of the Doctor we see after that point is actually the 12th version of the doctor, which would make Matt Smith’s Doctor the ‘final incarnation’ in that regard.
Now before your head explodes from all this exposition, let me simplify it for you here, if we are to take this theory into account, this would mean that at some point between the episode ‘Journey’s End’ and ‘The End of Time’ the doctor needed to have created a separate version of himself, unintentionally, while doing something unorthodox while regenerating, which we don’t see…officially that is
While it has been never stated in the show, or by the show runners, my personal theory is that the Valeyard is actually what the fans have come to refer to as ‘the meta-crisis Doctor’ a half time lord, half human clone of the doctor created by mistake by the doctor in dodging his regeneration, by pouring his regeneration energy into his spare hand (long story, don’t ask)
This version of doctor was shown as being more emotional due to being part human, and more angry and violent due to being born in the middle of a battle between the Daleks and the Doctor’s friends, and by the end of that episode (Journey’s End) he was left on a alternate universe Earth with Rose and her family, with it being implied that he and Rose would get together, since due being part human, he would still age like one and grow old with her (despite being only 20 minutes old at the time), and the original plan was to have that spin-off into a separate show about Rose and her family defending her Earth in ‘Pete’s World’ as it has come to be called in the show, but that project fell through, which was partly why a scene in which the main universe doctor gave his clone a piece of TARDIS coral (yes, the TARDIS is made of coral, even longer story) to make his own TARDIS out of was cut from the final broadcast
Now, the meta-crisis doctor has become one of the most critised elements for those who are not fans of Russell T. Davis’ writing when he was the show runner of the show, since it’s very fan-fiction-esce, and wish-fulfillment for the audience who are huge fans of the idea of the Doctor and Rose getting together, and sad to say, I am in that camp, however, I feel that, with some clever writing, the Meta-crisis doctor could be salvaged and become a greater part of the series lore, which is where this theory of mine comes into play
Now, as Trial of Time Lord stated, the Valeyard was created due to the 12th incarnation of the doctor ‘experimenting’ with extending his regeneration cycle, and as we have established, the version of the Doctor during the episode ‘Journey’s End’ would be the 12th Doctor chronologically, and you could consider accidentally making a clone of yourself from regeneration energy as ‘Experimenting’. He’s half human and that means he can’t regenerate (and unlike the 8th doctor, he doesn’t have the Sisters of Karn to help him out there), which would explain why he would want to steal the doctor’s remaining regenerations, and his more emotional and, arguably, psychotic tendencies could explain his grim and despicable disposition in ‘Trial of a Time Lord’, so on paper, this looks like it could all add up, but, there is a big hole in this theory, which requires major amounts of conjecture (something that the Doctor Who fanbase does quite a bit, actually).
The Master described the Valeyard as ‘all the Doctor’s darkest elements combined’, in other words, pure evil, and the meta-crisis doctor, while a bit genocidal, was by no means evil, plus, he looks nothing like Michael Jayston, the actor who played the Valeyard, so what could be explained to make the Meta-crisis match up with the Valeyard there? Honestly, there’s nothing I can say that will be backed up by actual evidence in the show, so this is where it gets into fan fiction territory (I’m joining you, RTD)
Let’s assume that when the Meta-crisis Doctor was left in his alternate Earth, he started getting restless, like any version of the Doctor tends to do when staying in the same place and time for too long, and while he still aged like a human, like I said earlier, he had only just been born at that point, so he could of out-lived Rose and her family by about 20-30 years if we take that into account, which would naturally be hard for him, and it would probably want to make him start again and escape his world and get his own TARDIS to forget about it all, and in the process of trying to force a regeneration through experimenting on himself, it could have accidently extracted all the good from him and made him into a pure evil version of his past-selves, which would explain his look and personality.
Now, as for escaping his parallel Earth, the doctor said that would be impossible due to the walls between dimensions being closed off, and only due to the reality of the Doctor Who world being warped due to Davros’ ‘Reality Bomb’ was it made possible again, now since that has happened, plenty of events could have caused the same kind of trauma to the source wall’s of the universe, the universe blowing up due to people trying to destroy the Doctor’s TARDIS, the time-space continuum being thrown into a blender by River Song refusing to shoot the Doctor, the Doctor himself being temporarily erased from history, or Galifrey being transported to another point of the universe during a Time-Locked point of history, anyone of those could have weakened the reality walls enough for the meta-crisis doctor to escape his universe, and into the main doctor’s one, and it has never been stated what has and hasn’t happened in ‘Pete’s World’, that has happened in the main universe (at least not in a full disclosure), so the Time Agents could still exist, as well as Captain Jack, so the meta-crisis doctor could have use a Vortex Manipulator to enter into the main universe, but got the timing off and landed in a point where the doctor was in his 6th incarnation, and thus decided to steal his regenerations and thus his life
Now admittedly this is, like I said, pure conjecture, plus it creates a massive paradox, since if the Meta-crisis doctor steals the remainder of the 6th doctor’s life, then that would mean that the doctor would never have tried to stop Davros’ and his reality bomb, and so the meta-crisis doctor would have never had been created, but then again, with all the other paradox’s that are trying as hard as they can to not destroy the entire universe in doctor who, another one couldn’t hurt, right?
Either way, this is a nice ‘fan-wank’ theory to chew on in think how the Valeyard could not only come back, but also David Tennant, and this time, playing an evil version of his previous role, which would be amazing, but, with the amount of plot points, characters and ideas that have been abandoned by the writing staff of Doctor Who, is unlikely that the Valeyard will appear again, but, with a new head writer coming in soon, who knows what might happen, and people have even theorized that he may show up again in this current series, but, until then, much like the unreleased Doctor Who cartoon (which you can watch a video I did on it here: https://www.youtube.com/watch?v=GbSF0Hvy7LI), we can only imagine if this would really happen, and what it would be like if it did.