Thirteen paces around the TARDIS, pondering where to go next. He just finished redoing all the mistakes he’s ever made. He’s decided to fix the biggest mistake he’s ever made. Not destroying the Kaleds when he had the chance.
Arriving on a bleak, empty Skaro*, just after the fourth Doctor has left. he sets up some argon explosives and detonates. In the resulting explosion, a large shard of metal hits him. Screaming in agony as he removes it, realizing that this is the end, he stumbles back to the TARDIS.
“DOCTOR!” booms a voice from nowhere. “IT IS TIME I TOLD YOU A STORY.”
“What? Who’s there?” asks Thirteen.
“I AM THE VOICE OF THE TARDIS,” replies the voice. “THERE IS AN OLD GALLIFREYAN TALE THAT STATES THAT ONCE A TIMELORD DIES PREMATURELY THE REMAINING REGENERATIONS ARE SPREAD OUT
EVENLY TO ALL REMAINING TIMELORDS.”
“And this helps me how, exactly?” asks a rather annoyed, and dying, Doctor.
“BECAUSE THESE STORIES ARE TRUE, AND SEEING AS YOU ARE THE LAST REMAINING TIME LORD, I HAVE CALCULATED THAT YOU HAVE 5269 REGENERATIONS LEFT.”
The Doctor then collapses on the floor. Screaming in pain, he regenerates.
“DOCTOR, I WAS CREATED WITH THE KNOWLEDGE THAT YOU WOULD STEAL ME**, AND I WAS ENTRUSTED WITH THE TASK OF TAKING YOU TO WHERE ALL INJUSTICES ARE HAPPENING FOR ALL ETERNITY. I WILL ONLY COMMUNICATE AGAIN WITH YOU WHEN THIS TASK IS DONE. YOU HAVE A LONG WAY TO GO, DOCTOR.”
“Wow, that was new, weird but rather enjoyable. Right, tea first, or coffee? One of both, then wardrobe and last but not least, off to save the universe!” a new Fourteenth Doctor says, to no-one in particular. “I have a rather good feeling I’m going to enjoy this.”
*Genesis of the Daleks
** Unearthly Child (I think)