In a Heartbeat
By TwinEnigma
Disclaimer: I do not own Doctor Who, nor do I make profit from this.
Warnings: Time Paradox oh shi-, spoilers for Parting of the Ways, Stolen Earth, Journey's End, Boom Town, Runaway Bride, Last of the Time Lords, Utopia.
It was an impossible wish.
Rose Tyler would not remember that, though, nor was she ever exactly completely aware of it during her possession by the Vortex.
The Doctor definitely didn't remember, in spite of his ability to read the lines of time.
The heart of the TARDIS is telepathic and reads all things, even between the lines and secrets, all those secret wishes. It is said that it can grant wishes, but like many wish-granting things, there are limits to what it can and cannot do. And often it is prone to taking wishes most literally, for it is also a computer – albeit a living one - and like all computers, logic is the premise of its construction.
A Slitheen wished for a fresh start and became an egg. Certainly, that was not her exact intention per se, but as far as wish-granting goes, she did get exactly what she wished for.
Rose Tyler's wish was a fair bit more complicated, consisting of several layers of wishes and wants.
She wished for the Doctor to be safe.
It destroyed the Daleks and guided them across time using the Bad Wolf. It put Donna Noble in the position to be dosed with Huon particles, so she would stop the Doctor beneath the Thames. It guided Martha Jones into his path, that she would walk the Earth to save him. It raised Jack Harkness from death so that there might be an Earth to ensure these paths would cross.
She wished to do something to help him.
It took the form of the Bad Wolf to act through her. Later, it called to her from the street, a siren song that found her taking his new, new hand once more. It guided their path and stranded her in another world so she would grow into a person who could help him again.
She wished to stay with the Doctor forever, to grow old and die with him.
It could not do that without breaking the previous two wishes and, by that point, it was being drawn out of Rose and into the Doctor, his wishes being entered like commands.
His wishes were the most complicated of all, in spite of their simplicity.
He wanted to live.
It made sure he did.
He wanted to help others, as the last legacy of his people.
It made sure he could help.
He wanted to be loved.
It made sure he was, by so many and in so many ways, as lover, as family, as a friend and as a hero.
He wanted to be like a human, more than anything, for all sorts of reasons.
It influenced his regeneration into the Tenth. It guided his hand to fall into Jack's care. It guided Rose, the Doctor and the Dalek to that one street to gather the Artron energy it needed. It called to Donna through the remains of the particles within her heart so that she would stay behind and used her touch to catalyze the reaction that would make him like a human.
In the end, only by granting his wish was it able to grant Rose's.
Rose and her Doctor would be safe in the parallel universe, as much as they were able to be, and Rose would help him, because now she could. They would grow old and die, because he was now like a human. He would live, be loved as a man, as family, as a friend and a hero, and he could still help others as a legacy of his people.
The original Doctor would also live and would come to love again. He would help others, as the last legacy of his people, and he'd become a little more human in spirit.
It was an impossible wish, but the heart of the TARDIS thought it had managed quite well, all things considered, and settled back into the console to watch their wishes unfold from the slow path.
AN: It makes sense to me at any rate.