As the Doctor goes on, he grows in arrogance, until he views himself not as the one survivor of the Time War, but as its winner--entitled to do as he pleases, at his own whim, to decide the fates not just of "the little people," as he actually calls them, but of heroes:
The Time Lord Victorious is the Doctor manipulating events to suit his will, not unlike his own nemesis the Master. And sure enough, when we next encounter the Master, he and the Doctor draw nearer still, opposite sides of the coin. Just as the Doctor has taken on the Master's arrogance and recklessness, so too the Master dies saving the Doctor:
Davies ends Tennant's tenure, and his own, by returning to the projected, but never filmed theme of what was meant to be Jon Pertwee's last episode, The Final Game. He does not, in fact, go as far as that story was intended to:
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.Davies expressly rejected the "brothers" theory in The Sound of Drums, but consider a moment: the Doctor has seen his own darkness, and, in revulsion at his own arrogance, is prepared to die to stop the corrupted Time Lords under Rassilon destroying, well, everything. The Master makes the same choice, first telling the Doctor to "get out of the way," mirroring the Doctor's own prior warning to him.
It is hard to imagine a story that brings back the Master without undermining that ending, and though I have always enjoyed the character, I would hate for that to happen. Unless the integrity of the character development can be respected, and a new kind of story told, I'd prefer Moffatt leave the Master in peace.