Well you're just going to have to read this to the end to find out. Don't know if I'll develop this into a full blown story, but the idea would make an interesting bit of fan art (if I could only draw .....)
Regeneration
Lightning flashed through the sky as a hot rain fell upon the scorched ground. The falling liquid hissed as it touched the hot earth that still glowed with a dull red heat, still warm from the hot liquid rock that had rained down from the numerous now quiet volcanoes. Somewhere in the middle of the hellish field a small raised spot still sported moist earth, untouched by the demonic fire that had raged all about it. Standing in the middle of that oasis stood a strange looking woman. She wore a gas mask to protect her from the sulfur fumes that were still bubbling out of the ground.
Through the thick clouds of ash still carried aloft in the wind, the light of the planet's three moons was barely visible. An eerie complex of shadows were cast by the light of the trio of satellites, and their light would have made the features of the woman visible to anyone that might have been close enough to see them. She was dressed in a black and red dress, stiletto heels, and wore a dark veil over her face. The sounds of her breath through the filter mask that protected her lungs from the toxic atmosphere made her sound like a machine, a female Darth Vader.
She held a long slender pen like device in her right hand. As she activated it, three small balled tips extended from the end, and small bright sparks of electricity jumped between their chromed surfaces. The device emitted a powerful pulse into the atmosphere that cleared the planet's thick atmosphere and traveled at relativistic speed through space.
The warning klaxon sounded and multiple indicators on the Tardis control panel started flashing brightly. The timeship flew out of control as the homing beacon activated the automatic controls. The Doctor was thrown out of the comfortable chair he had been sitting in several rooms away and his head hit the floor hard enough to raise a small bump on his forehead. He somehow got to his feet and tried to make it to the control room against the tremendous G forces of acceleration that the Tardis was now undergoing. It took him almost two minutes to reach the main panel.
“What's going on old girl?” he asked the machine, not expecting an answer. The overhead video screen was flashing “EMERGENCY 401”, and the twin sirens were still blaring away. The Doctor found the switch to silence the Klaxons and then began to engage all of the emergency override switches. The machine stopped bucking like a bronco, but it was clear that he was not yet in control of his destination.
“What emergency homing beacon?” he asked himself as he looked at the readouts.
The doctor knew that every Tardis had been built with a device that would engage when a distress signal from another Tardis was received. Only another Time Lord could send such a distress signal, and the Doctor was the only member of his race left alive, at least that is what he had believed for nearly a score of Earth years now.
There was no mistake about this however. His Tardis had now locked its course onto the distress beacon sent by another Time Lord. He had no choice in the matter, he had to respond. The machine's velocity was now slowing, the Tardis was on a direct descent towards whatever planet the distress signal had been sent from.
The dark woman stepped back to the edge of the oasis , her heels just missing the still red hot Earth beyond the circle within where she was standing. Just a meter or two away from here the air started to shimmer as a grinding sound became louder and louder. The noise resembling a recording of a siren being played backwards on an old fashioned tape recorder at the wrong speed suddenly stopped as a tall blue box materialized in front of her. She aimed the pen like device at the door of the machine and the lock opened up. The woman quickly pulled the door open and stepped inside, closing the door behind her.
The Doctor stood motionless as his uninvited guest slowly walked towards him. She slowly removed her veil and her gas mask to reveal the face of a rather attractive woman. She was tall, almost a head taller than the doctor.
“About time you showed up!” she said. “I was beginning to be overcome by the poisonous atmosphere outside.”
“Who are you? And how did you summon me here?” The Doctor demanded.
“You know the answer to that.” She replied. “I am a Time Lord, just as you are. I thought I was the last one, just as you thought you were the last. I was abandoned on this planet, sent here though a time vortex. In two more seconds I'd have been dead. You were cutting things a bit close.”
“Well the old girl isn't as spry as she used to be,” The doctor replied, “But she still manages to get me about.”
“Old girl?” the woman asked.
“My Tardis,” the doctor replied.
“And you are?” the woman asked.
“I'm the Doctor.” the doctor replied.
“That's what I was hoping,” she laughed removing a small box from her pocket.
“It's time.” She smiled, “For us to become as one.”
She opened the box, and removed two lead tubes. She opened both of them and poured their contents into the palm of her right hand. Each of the two tubes contained a glowing powder, when the contents of both tubes combined the powders glowed brighter and filled the Tardis with a blinding light.
“NO!” the doctor yelled, “What are you doing? That's Kronoactive element.”
“Correct Doctor!” the woman laughed. It's too late, we've both been exposed to the reaction of the Krono radiation. It won't take long, don't worry.”
“Why?” the Doctor asked.
“Because it's the only way. We must both regenerate together. We must become as one and restore the Time Lord race!”
The doctor felt the effects of the radiation almost at once. His head began to spin and his legs became as rubber. The woman too fell victim to the poison. Both Time Lords lay on the floor of the Tardis within inches of each other. Their bodies began to glow as the regeneration effect was triggered.
The Doctor slowly came back to his senses. He knew he'd changed, he'd been through the process of regeneration too many times before, and he still didn't know quite what to expect. He got up and looked down at himself. He was dressed in black trousers, wearing black leather shoes and stockings. He had a white shirt on over which he was wearing a black cape like top coat. He had a red ribbon bow tie around his neck.
The doctor walked towards the mirror to see his new face. He was taken back for a moment at what he saw. His full head of hair was half dark, and half pure white. He had a surgical scar running diagonally across his face, the skin on the left side of which was a darker shade than the right.
He saw the woman who had done this to him still lying on the floor, only now waking up. She had transformed into a young child barely a third his height. She was wearing a short pink skirt held up with suspenders over a light colored blouse. Her brown hair sported four pink bows. The girl looked up at the doctor and then down at herself as if she was in a state of shock. “Acchonburike” she muttered, “This hasn't worked out quite the way I expected!”
I'm on Fanfiction.net as Tetsuwan Penguin. Please check out some of the other stories I've written! 
it may take me a few days to complete though.
