Prologue: A decade after an AU Deathy Hallows.
Begins: Summer before AU Fourth Year.

Alterations from Canon:
Just think how thin, flimsy and fragile the cause and effect that leads Harry, Ron and Hermione from being clueless campers to retrieving the Cup horcrux from the Lestrange Vault. Had Harry not blurted the word 'Voldemort' and triggered the Taboo, had they not been taken to Malfoy Manor, if they had not already acquired the Sword from Snape for Bellatrix to get upset about, had they not gotten Bellatrix's wand from the fight, had they not encountered any Goblin to help their conspiracy...

So what if indeed Harry hadn't triggered the Taboo? What if Hermione had silenced him before he got the name out? What if Ron said something and interrupted him? What if the argument they were having happened a day earlier or later? Or they were taken to somewhere other than Malfoy Manor? Instead leaving them to stumble about ineffectually as the war dragged on...

Undecided. Author's perogative.

As a Harry-in-an-AU fic the canon characters may differ and their histories/relationships likewise, so typical pairings and their significance are less relevant or likely.

It's All Hallow's Eve 2008 and standing in the ruins of the Potter's Godric's Hollow home Harry has just ended the Blood War for the final time by destroying Voldemort for good. Unfortunately it's too little, too late. The whole of Magical Europe is a blighted wasteland of untamed magical storms, eldritch abominations and cursed ruins. Millions magical and muggle are dead.

A stalled Horcrux hunt delays the defeat of the Dark Lord for years as the death toll skyrockets on both sides and the war spread across the entirety of the continent. Only the discovery of the soul-fragment in Harry's curse scar and the scrying of an Unspeakable allows the remaining horcruxes to be found and destroyed.

Barely started on what is sure to be the first of many bottles of cheap liquor Harry is dragged away by the Unspeakable to see what the Dark Lord was doing at Godric's Hollow. A ritual, the Unspeakable declares, that appears to be intended to take a person's mind and memories and send them back in time. No doubt the Dark Lord chose this spot so he could go back 27 years and avert his first fall from power.

With warnings that modifying the ritual, which the Unspeakable only partially comprehends, could blow-up in their faces Harry, with nothing to lose, rolls the dice. The target is altered to sometime during the summer of 1994, a time when Harry feels he would have sufficient opportunity to destroy the horcruxes and end the Dark Lord at the culmination of the Triwizard Tournament.

It is almost immediately apparent that the ritual didn't work as intended or that the Unspeakable had misunderstood the purpose of it when Harry awakens in the foyer of a home he has never before laid eyes on.

Finding a woman with thick, black curly hair lying face down next to him he liberates her wand (upon discovering none on himself) and has barely confirmed her to be alive when the sound of screaming coming from elsewhere in the house reaches his ears. Soon he finds a room with a woman bound to a chair, beaten and crying while a Death Eater stands over a moaning and writhing man on the floor, threatening to use the Cruciatus again. Battle reflexes and instincts honed by a decade of front-line fighting aids Harry in defeating the extremely combat competent Death Eater.

The true severity of his situation is revealed to him when he sufficiently heals the face of the woman enough to recognise none other than Lily Potter. Pushing away his shock and realisation he performs field triage on both her and James Potter before calling St. Mungos on the Floo and summoning a team of emergency healers. Pausing only to direct the new arrivals to the three in the room and the woman in the foyer he calls the DMLE and soon Mad-Eye Moody, Tonks and Shacklebolt arrive on the scene as well.

It doesn't take long for the evidence or Harry's behaviour to trigger Moody's paranoia. Harry recognising Crouch Jr. when the mask comes off, the remains of the battle clearly revealing the magic used to Moody's keen observation, the details of the medical treatment that Harry relates to one of the healers. The final straw is when healers return to the room with the woman from the foyer.

There's no doubt who it is the moment Harry sees her face. Bellatrix... Black apparently, as Shacklebolt identifies her, or as Tonks calls her 'Aunty Bella'. Bellatrix's aghast reaction at seeing Harry alive, declaring she saw the Death Eater hit him with the Killing Curse sees Harry dragged off to the DMLE. In Moody's estimation too much doesn't add up and he's going to get to the bottom of who is responsible for what.

The Fallout:
Sitting in the interrogation room at the DMLE Harry is forced to dance carefully to avoid outing himself even as he tries to glean what facts he can about this unknown world he has found himself pitched into. One of his missteps happens early on when he states he is 'Harry James Potter, son of James and Lily Potter, born 31st of July 1980', only for Tonks to uncertainly inform him that he was born on the 1st of August.

He wasn't born 'as the seventh month dies' in this universe, he's not the Boy-Who-Lived. The Dark Lord did die on All Hallow's Eve 1981 but while he was trying to kill the son of Sirius Black and Marlene McKinnon. He also learns that at least the Unspeakable got one thing right, it is the 2nd of August 1994.

Throwing their own regulations concerning the interrogation of minors in their faces sees Harry put into a holding cell until his parents can come or a legal advisor is appointed to him by the DMLE. His sleep is interrupted by a dream about how his counterpart in this universe died when Crouch Jr burst into the Potter Estate apparently having followed Harry and Bellatrix back from the marketplace in Godric's Hollow, just a short walk from the house. Harry realises he's effectively taken residence in the recently vacated body of his counterpart, perhaps only minutes after the Killing Curse 'emptied the vessel' so to speak. Occlumency reveals that locked away in this body's brain are most if not all of the former occupant's memories, largely undisturbed by the brief oxygen-deprivation of the body's short death.

From these memories Harry begins to gradually gather an understanding of this new world he lives in and realises that while circumstances have changed there's every indication in the memories of native!Harry's adventures with the Boy-Who-Lived, Sirius' son, that the Dark Lord is working to return in this world as well, so the mission is essentially the same.

Some Ideas:
To throw against the wall, so to speak.

Sirius and Marlene's pregnancy wasn't planned but rather an accident after a bit of 'life-affirming fun' following a battle, Marlene's contraception was negated by a healing potion administered in the aftermath. Their relationship grew but was cut short by her death while facing off against Voldemort to defend their son.

Two years later Sirius did marry, a Veela he met in France, where he took his son to get away from the press, ongoing threats and his harridan mother. Rinette is actually the cousin of Fleur and Gabrielle's mother, Appoline. Shortly after the Black family grows with the addition of twin daughters.

After years of false starts and lost hope Lily is actually pregnant and Crouch's curses don't appear to have done any damage to the foetus either.

Harry eventually determines Crouch's interest wasn't in trying to get to Sirius and his family, including the Boy-Who-Lived, but wanted to get into the Black Estate, Grimmauld Place, because he had found Regulus' note with the fake Locket horcrux and was seeking to find where it may have been taken.

In this reality Crouch Snr was one of the very brief Ministers during the war and likewise was assassinated, though he didn't die immediately, relinquishing his office he agreed to his wife's plan to break their son out of Azkaban, force him to swear an Unbreakable Vow to obey his mother (whose eventual death freed Jr) and for Crouch Snr to take his son's place under polyjuice.

Narcissa married Rodulphus Lestrange and is this universe's mad-woman, nightmare, Death-Eater and lunatic while Bellatrix married Lucius Malfoy only to be cast aside in favour of a new wife when tests proved her to be barren. Her supremacist beliefs laid bare for what they were, with the Black family refusing to take her back, she goes to the Order, swears to oppose the Dark and discovers Snape's membership. This eventually leads a relationship to develop as Snape was the only Death Eater to show her kindness in between the discovery of her infertility driving Lucius' ire and when he finally divorced her and cast her out. Taking up the role of a Healer for the Order she spends a lot of time with Snape brewing medical potions and over time he begins to see and favour in a gradually recovering Bellatrix the same qualities that drew him to Lily.

Hermione is a Ravenclaw in this universe, as is her best friend Daphne Greengrass. His counterpart saved her from the Troll in First Year too and so they have a degree of familiarity but not quite friendship due to being in different houses. A further source of tension is that she's also quite proficient on a broom but has a total lack of interest in Quidditch while native!Harry was obsessed with the sport. Harry's new personality and behaviour flusters her substantially due to her existing attraction towards the previous Harry but without many of the detractions that held her interest back. Daphne is mildly jealous, not yet self-aware of her attraction to her best friend. Hermione is also even smarter in this reality, a borderline polymath who has already been published in magical journals for improving several spells and potions.

This universe has no Ginny, nor a Ron. Instead there is Matilda Weasley, born just about the same time as Ron would have been and after getting the desired daughter Molly stopped having children. Matty is an artist, often covered with smudges, like her counterpart, but usually charcoal and paint. She's an avid Harpies fan and idolises Katie Bell wanting to become a chaser on the Gryffindor Quiditch team more than anything.

Remus isn't a werewolf. His mother was bitten by Greyback while outside looking for Remus that night, past his curfew. With his father dead from an accident his mother struggles to look after Remus and hold down a job with her condition. When he is 15 his mother's condition is discovered and she loses custody and the Potters take him in. He gets emancipated and learns to become an animagus (as do the other Marauders), a wolf by coincidence, so he can go home to his mother. During the first Blood War she is murdered by Yaxley who as an Auror claimed he was defending himself from a 'Dark Creature'.

Native!Harry was smitten as hell with Tonks when he was about 10. Tonks always found it amusing to flirt and tease him, even years later. The tables turn significantly with a Harry who is mentally twice the age and able to give as good as he gets.

Daphne's father is the Minister for Magic, a semi-light neutral and more of an economist who has been breaking up stagnation caused in large parts by anti-competitive monopolies and pureblood hiring preferences favoured by the Old and Dark families. Malfoy is running for office though and the marginalised families, Dark and the just plain greedy, are gradually building a politcal machine more than willing to dirty their hands to secure the next election.

Fleur and Gabrielle have another sister, Claudia, born between the two of them and the same age as Harry. Having met the Potters' through Sirius' marriage to Rinette the sisters know Harry and despite having somehow earned Fleur's ire Harry finds himself the object of pursuit by Claudia when she arrives to support her sister at the Triward Tasks.

Sirius' son (the Boy-Who-Lived) is almost as much of a lecher as his father but has already landed himself 'the best bird' in his estimation, his evaluation mostly based on Lavender's bust. Native!Harry was no less like his own father having been to Hogsmeade with a different witch on each weekend the year before and kissed almost every one of them afterwards. This year many more of the witches in their year turn their attentions and interests from the apparently taken BWL to his best friend, Quidditch-seeker and ladies man side-kick. Which is all sorts of awkward for Harry whose mental age makes it all very confusing for him.

I've always loved AU fics that change almost everything without really introducing very many new things, instead just rearranging what's already there in subtle ways and into new patterns so everything fits together still but in a different order.