The Final Game (unproduced TV story)

✅ The Final Game was an unproduced serial intended to end Season 11. It would have revealed that the Doctor and the Master were actually brothers or the latter Time Lord was the former Time Lord's darker personality. This story originally ends with the Master's Roger Delgado incarnation sacrificing himself, then the Third Doctor regenerates. But the serial was never produced due to Delgado's sudden death. Instead Planet of the Spiders replaced this story, and Delgado's character was shelved until The Deadly Assassin, which recast the Master as Peter Pratt, in a decayed state.

Legacy
Several elements from this episode have been recycled throughout the years:
 * Two elements were recycled in TV: Logopolis:
 * The Master being the Doctor's dark half was alluded when the Fourth Doctor implied this to Adric and Nyssa.
 * The Master causing the Doctor to regenerate occurs when the Master's Anthony Ainley incarnation caused the Fourth Doctor to regenerate.
 * Their possible fraternal relationship was teased in TV: Planet of Fire by the Master's Ainley incarnation to the Fifth Doctor and Peri Brown (but TV: The Sound of Drums later dismissed this theory after the Tenth Doctor was asked this question by Martha Jones).
 * The idea of a corporeal 'dark side' was used in TV: The Trial of a Time Lord, when the Master's Ainley incarnation revealed that the Valeyard who had been prosecuting the Sixth Doctor was in fact a manifestation of the dark side of the Doctor's own personality.
 * The original ending occurs in TV: The End of Time when the Master's John Simm incarnation sacrifices himself to save the Tenth Doctor from Rassilon. However, the Doctor regenerates soon afterwards due to radiation poisoning.