Doctor Who: 10 Theories About What Happens To The Fourteenth Doctor
9. He Becomes The Valeyard
1986’s season-long story The Trial of a Time Lord introduced the Valeyard, an amalgamation of the Doctor’s darkest qualities who fits somewhere between his "twelfth and final" incarnations.
At the time it had been established that Time Lords could regenerate a maximum of 12 times – and as such, “final” meant thirteenth. These days, in light of the Metacrisis and War Doctors, plus the Doctor’s second regeneration cycle, things are a bit more complicated.
However, if we take “final” to mean the Doctor’s final ever incarnation (including all regeneration cycles), the Valeyard could come about at any point after the Tenth Doctor’s regeneration in The End of Time. So is the Fourteenth Doctor a likely candidate?
On the one hand, it’s hard to imagine such a happy and fulfilled Doctor going rogue, but then again, he could become bitter and vindictive in old age, especially once his newfound family is no longer around. The last time a Tennant Doctor was on his own, he wasn't exactly well-behaved.
And wouldn't it be an amazing hook for the show if the Doctor was the villain of the season? We've seen plenty of multi-Doctor stories before, but never quite like that.
Plus, David Tennant plays evil really well, as we saw in Marvel's Jessica Jones.