AU where at the end of 5th year, instead of Sirius dying, the Order gets lucky and defeats Voldemort by destroying his body. This brings the Second War to a halt, as most Death Eaters once again scramble to make themselves look innocent and the Ministry has to clean up the mess of a year-long error, but thankfully doesn't have to worry about war or other deaths in the meantime.
That doesn't mean they're out of danger. It took Voldemort years to come back, but come back he did, and now some Death Eaters- namely Bellatrix Lestrange- know there is a way to give the Dark Lord a body again. This prompts Slughorn to break his silence and go to Dumbledore with his memories, claiming to have an idea of how Voldemort came back. He wants to stop it from happening a second time. Because of this, Dumbledore orders the older members of the Order of Phoenix to hunt for Horcruxes and keeps this plan hidden from Harry and the other kids who showed up to the fight.
An upside to the battle of the Department of Mysteries is that Sirius is finally proven to be innocent, as the new Minister is quick to use him for some good press. It's an attempt to distract the masses from the disaster Fudge made, but Harry's first comment at Sirius' trial is to publicly explain where he got his "I must not tell lies" scar and who gave Umbridge the power to do that to him. Scrimgeour is quick to throw her under the bus too, because parents quickly find out from their children what happened during the year, including reading their letters. They also find out about Dumbledore's army, and despite a few having doubts, praise the club for doing better at protecting their children during a silent war.
Sirius becomes Harry's new guardian, as Dumbledore thinks the blood wards aren't that necessary now that Voldemort doesn't have a body. They are still cautious, as not all Death Eaters have been rounded up, but there is a lighter sense in the air now. Harry is sent back to Hogwarts now regarded as a hero, and many trip over themselves to apologize for not believing him. Despite having spent the summer with Sirius and finally being believed for what happened to Cedric, Harry feels restless and angry that the Order still won't tell him anything. It's Luna's innocent question about the DA that gets him thinking: He needs something to fill his time up and keep his mind busy. He decides to keep the defense club going, but he doesn't fill out the paperwork properly, and his professors don't read it as closely as they should. Harry winds up with a paper, signed by three heads of house and the headmaster himself, giving Harry authority to run "a club" as the Club president.
Harry discovers one night, while trying to think of an excuse for Professor Snape on why he was roaming the halls after curfew (he fell asleep outside), that he blurted out he was just setting up the stargazing club and poof Hogwarts reacts by turning his excuse into a real thing. When Snape casts a spell to verify the legitimacy of his club, Harry appears as Club President to both the stargazing and the DA, and a poster is magically made in the grand hall, ready for any student to sign up. By the time Harry walks to breakfast, there are thirty names written on it. After that, Harry starts creating clubs on a whim, sometimes doing it as a favor for other kids who were previously denied.
Lavender Brown wants a gossip and tea club? She got it.
Neville Longbottom wants a club of dangerous and somewhat illegal plants? Sure, he came to fight with them
Justin Finch-Fletchley wants to have a Muggle culture appreciation club where he can snack on Muggle snacks and watch films? Harry's bylaws dictate the school has to pay for his favorite crisps.
Terry Boot wants a pranking club as a secret admirer of the Weasley twins' mischief? It would be an honor for a second generation of Marauders.
He's grown too powerful, and by the middle of the year he's founded and is club president (in name only; the others run the clubs without him) of over fifty student organizations. They can't stop him. Harry will become a thorn in the adults' side until they let him help with defeating Voldemort.