7. The Valeyard Is Coming
According to previous episodes, the time of the darkest incarnation of the Doctor is coming. If that isn't resolved during the 50th anniversary episode, then during the 12th Doctor's run he can encounter this sinister version of himself during his run. The Valeyard initially appeared during the 6th Doctor's run, to prosecute him for committing genocide. It is later revealed that the Valeyard was actually a future incarnation of the Doctor toward the end of his regenerations, who had tampered with the Matrix of Gallifrey to create false evidence to convict the 6th Doctor and claim his remaining regenerations for himself. The Valeyard failed in his attempt, but it does seem ironic that he accused the Doctor of genocide, given the Time War happening later in the modern series. Maybe the Valeyard knew what was coming, tampered with the Matrix so that the Time Lords never knew of the coming Time War? Despite the Valeyard's plans being ruined, he got away by blending in with the rest of the High Council and hasn't been seen since.