After the devastating Battle of Hogwarts, Harry and Fleur barely escape to continue the war effort from the underground. But no matter what they try, they simply cannot defeat Voldemort. A few dead Death Eaters here, a burnt down mansion there, but they can't stop his extermination of the Muggleborn any more than they can bring back the dead who fought at their side. In a desperate move, they prepare a time travel ritual, but before they can complete it, Harry is killed. Fleur goes through with the ritual out of pure spite, and with a burning desire for vengeance.
Originally, it was supposed to take them both back to 1993 (capture Pettigrew, free Sirius), but because there's only one person to take back, Fleur wakes up in 1982. Her first order of business is to Kidnap Harry. The second one is to kill every single Death Eater she can find. She cannot, however, take care of all Horcruxes since she can't break into Gringotts for example. She becomes feared as a new dark wizard, thought to be a vigilante Muggleborn, all while happily raising Harry. He might not be her Harry, but she has seen the scars the Dursleys left on him (emotional for the most part).
Bonus points if she's super critical about Harry's girlfriends.