When it comes to works in a series — novels, films, television, it doesn’t matter which — I like playing the how-would-I-end-this game. It’s the fiction-writing equivalent of that improvisational drama exercise where you have to construct a skit around four or five random objects drawn from a grab bag (an argyle sock, a popsicle stick, an outdated guidebook to Tblisi on Five Dollars a Day, and a fishing lure with the hook snipped off…you have ten minutes to brainstorm with your group and then we’ll begin) –the idea is to get from where you are to an acceptable victory condition in five moves or less.
It’s an amusing game; but while I’m playing it I have to keep a firm grasp on the fact that the story I’m ending in my head is, despite any surface resemblances, a different story than the one the author is ending.
Dr. Doyle's Blog 
Here’s how I think the Harry Potter series should have ended:
Uncle Vernon and Aunt Petunia were really very powerful wizards, living under deep cover. (Vernon had been Head Boy at Durmstrang; Petunia had graduated at the top of her class at Beaubaton. They’d been slotted in because, since they hadn’t gone to Hogwarts, Voldemort and his Death Eaters wouldn’t recognize them. Dudley was also a very talented wizard. He was given his oafish appearance by means of a polyjuice potion and was being home-schooled.)
In the action/adventure climax, Harry has retreated to his last refuge, the house on Privet Drive. Its under siege. Voldemort comes strolling up the walk, enters, is about to do his nefarious worst, when Vernon pulls out a Remington Model 870 Express tactical shotgun and says, “Ever hear of the Misuse of Muggle Artifacts?”