[Be sure to read the comments: I wrote the first few of these here, but a lot of people have contributed some truly funny endings.]
The end of the Harry Potter series, as written by
Mario Puzo (The Godfather) :
Replacing the fallen Dumbledore as head of Hogwarts, Professor Minerva McGonagall proves to have balls of steel. In a well-coordinated series of carefully-timed attacks, the Order of the Phoenix kills every single Death Eater in a single night, settling all accounts.
John Grisham (The Firm, The Rainmaker, The Pelican Brief) :
Over the course of the book, Harry becomes disillusioned with the wizarding life as he realizes that it’s just endless conflict in the service of his corrupt and power-hungry masters. He and Ginny change their names and assume new identities so they can leave the wizarding world and live happily ever after.
George Lucas (American Graffiti, Star Wars, Star Wars, Star Wars, Star Wars, Star Wars, Star Wars) :
The battle between Harry and Voldemort comes to a close with an exciting magical glowing sword fight in the never-before-seen high-tech part of Hogwarts during which Voldemort reveals that he’s Harry’s real father. Then the Death Star blows up.
Joe Haldeman (The Forever War, Forever Free, Worlds Enough and Time) :
During the final confrontation, Neville Longbottom reveals unsuspected powers when he kills all the Death Eaters all over the world, including Lord Voldemort, by making them blow up into steaming bloody chunks. Neville explains that he is God The Creator Of The World in disguise, and then he blows up all the muggles in the world. God-Neville then proceeds to blow up all the all the wizards and witches in the world except Harry. Then he blows up Harry.
Dean Koontz (Bestselling Thriller Author) :
While Voldemort is preparing his final assault on Hogwarts, Arthur Weasley uses his knowledge of muggle artifacts come up with a plan to defeat Voldemort. He and Ron work desperately to connect the flue network to a fireplace in the house of a major Columbian drug lord. Harry and Hermione go through, stun everyone in the house, and steal all their Uzi submachineguns. When the Death Eaters arrive at Hogwarts, members of the Order of the Phoenix gun them down.
Joss Whedon (Buffy the Vampire Slayer, Angel, Firefly) :
As the conflict with Voldemort comes to a head, Ron Weasley is suddenly and shockingly killed. Hermione responds with steely determination, joined by Luna Lovegood, who turns out to be a rare witch who has super-powerful martial arts skills. While Harry tries ineffectively to help—often with comic results—Luna kills the Death Eaters with Unblockable Scorpion Kicks and Hermione defeats Voldemort in a head-on battle of magic.
Tom Clancy (The Hunt For Red October, The Sum of All Fears, Rainbow 6) :
All seems lost for Harry until the Voldemort problem is brought to the attention of American President Jack Ryan, who sends the Enterprise Carrier Group to defeat the Death Eaters in a series of air strikes. Distrustful of the Ministry of Magic, Harry Potter defects to the United States where he helps Ryan get elected to his fourth term as President.
David Chase (The Sopranos) :
Discouraged by Lord Voldemort’s tiresome battle against Harry Potter, Lucius Malfoy makes peace with Harry Potter in a bid to take over the Death Eaters for himself. Meanwhile, Alicia Spinnet spots Lord Voldemort in Diagon Alley and catches him by surprise, killing him easily. Through a strange series of random events, all Voldemort’s horcruxes are also destroyed. Thinking the danger is over, Harry, Ginny, Ron, and Hermione make plans to meet at the Three Broomsticks for butterbeers. Just as the last of them arrives