Written before 'Children Of Earth'. Having said that, everything after the first section is pretty canon-accurate.
Gwen has a child. Rhys has a beer belly. Ianto has a caravan. Myfanwy has a mate.
Jack takes great pride in all of these things. They belong to his family, they are his by definition. His delight is tangible when he does the school run, picks up another six pack of lager from the supermarket, spends the weekend in a muddy field or hunts out the best place for a pteranodon nest.
There are no secrets among the four (five?) of them. There are no personal possessions, no outside acquaintances. Any old school friends are introduced as such and become mutual by extension. One person shopping no longer buys food for those in their own household. They buy for four, and they do it without thought. The only thing kept separate is their respective flats.
None of them go home much, anyway.
Jack and Gwen are laughing at Mikel's pitiful attempt at paperwork. They often do this, crowd together round one side of Jack's desk to laugh at those working below. They talk of Gwen's - Torchwood's - child, now at university, and they talk about the past. The good old days of Japanese and Cockney. The good but slightly younger days of a Welsh archivist teaching Cardiff geography to a native lorry manager.
The rest of the team have become used to their little games. They have also become used to the weekly ritual, where both leader and deputy depart to God-knows-where only to return two hours later with identical drops of water making a bid for freedom from their eyelids.
Only Jack and Gwen know of the small cemetery where Rhys and Ianto lie side by side with Tosh and Owen. And only Gwen knows that the empty space on one side is reserved for an ex-police insomniac who has no intention of kowtowing to eternal sleep any time soon.
Only Jack knows that he clutches her hand that little bit tighter in the vain hope that the pressure of his faith will keep her awake.
Fifteen hundred years
Jack is still at Torchwood. He keeps the same photograph on his desk, framed with anti-fade technology that isn't human now, and certainly wasn't when he 'acquired' it. In the photo, Tosh leans in to Owen, Gwen throws an arm around both Jack and Rhys, and in a rare moment of abandon, Ianto jumps across the backs of all five, grinning broadly as he defies gravity atop five pairs of shoulders determined to unseat him.
Every time Jack can't remember why the human race is worth saving, he looks at the photo and forces himself to stare at each face in turn. Ianto receives the shortest amount of attention, simply because he died in too painful a way to provide any sort of justification for apocalypse aversion.
Gwen, he studies the longest, simply because she was optimistic to the last. Went out from natural causes at eighty-two, longest serving Torchwood officer ever. Well, longest serving human. Of course by the end, Jack was forcibly removing her from field work, but she fought him every step of the way. And so he fights.
He still visits the graveyard. Every week, just like always. But a sign has appeared declaring a new development project, and all of Jack's horses and all of Jack's men cannot stop it.
When the government outpost is built, Jack continues to lay a bunch of flowers on the same spot every week. He hates each and every person who does not ask him why.
Five thousand years
The Rift closes up. A mix-up with an energy-gathering supercharger and an impromptu visit from Jack's Doctor ensure that Torchwood Cardiff is no longer a necessity. In any case, First Contact has been made a thousand times over and Jack's beloved Hub has become little more than an interstellar Customs station.
He doesn't look back. The place is too wreathed in Owen's dry wit and the beeping of Toshiko's machines, too laced with Ianto's cramped handwriting and Gwen's scrawled messages, to be anything other than heartbreaking. So he marches out with his coat and an unused hat, the one Suzie bought him on his 'birthday' all those years ago, the one he swore he'd never wear.
Before he leaves, he picks up Tosh's glasses, Owen's stethoscope, Ianto's stopwatch and Gwen's engagement ring. Then he opens the roof, watches Myfanwy's last descendent spiral up to the stars, and switches off the light.
Five million years
This is not how Jack saw his future.
He is surrounded by fourteen machines beeping in fourteen different patterns, every single one failing to discover his secrets. This week's have-a-go evil genius is attempting to decipher his biology by… actually, Jack isn't sure how. All he knows is that if the guy's plan is to drive him insane through beeping, the world is probably doomed.
When he escapes - again - he takes the data with him. And when the Doctor comes to call, the two of them attempt to work out why Jack's aging process has suddenly begun.
They fail just as badly as the original machines.
Fifty million years
Jack has decided that aging is worse than immortality. An accident involving a scythe and three fairly surprised Slitheen has left him as a head. A fully-functioning and rather handsome head (if he does say so himself), but a head nonetheless.
The grey hair he could deal with, but being unable to regrow his body? Not good.
And worse, his life signs are fading. There was never any heartbeat to begin with, what with there being no heart, but all those spikes registering his brainwave pattern have been peaking lower and lower.
Suddenly, Jack understands what it feels like to be afraid of death.
It feels like being alive.
Five hundred million years
There is no-one alive who remembers Jack Harkness.
The being that once was Jack decided that it would be better if there were no awkward questions - such as why his head was hooked up to a life support system. And why that head was still alive with no discernable body.
So he changed himself; shrouded his features in the smoke of his life support and made a very public name for himself as someone else. That name was a joke, after a fashion: it reminded him of where he came from whilst completely obscuring his origins from anyone else. Irony, or something like it.
The Doctor still visits, but he always calls him Jack. New face, new companions, but that one thing stays the same.
The Face of Boe wonders why his name makes his oldest friend laugh.
Five billion years
The Doctor has long since stopped calling, On his final visit, he gave Boe a message to pass on to his younger self.
Boe chuckled quietly when he recognised the words from a long-ago trip to the end of the Universe as his own.
He settles quietly in a darkened corner of the world to wait for the end.
Five billion and twenty-six years
A Doctor with a long-departed face sits with him as a Martha gone before they ever met keeps her distance.
He whispers his last and closes his eyes to see two hands reaching out of the darkness. One is large, calloused and strong; the other is soft, delicate and strangely missing a wedding ring.
His final words, he thinks, are not only for the Doctor.