Any James Bond movie:
The villain finds a copy of the evil overlord manual http://www.eviloverlord.com/lists/overlord.html and shoots James instead of setting up an elaorate Death Machine.
Godzilla (Sony remake): The overgrown iguana chomps down the taxi containing our “heroes”, then gets blasted with a military nuke. It then mutates into the real Godzilla (the Toho classic), then destroys the ntire world in a massive fit for being stuck in such a lame-ass movie to begin with.
Attack of the clones: when Anakin marries Princess amidala, an alien from alien bursts out of her, claiming to be what he just married.
Would be somewhat funny, though.
Actually, those would make rather interesting novels to read…
Grease: Danny ends up with Rizzo. Sandy with Kenickie, down under. Doody, Sonny and Putzie start the first Le Bare. Frenchie, Marty and Jan become an eternal threesome. Somewhere in all this madness is Frankie Avalon in ALL black. 
Citizen Kane
Reporter: Hey! Look at this sled! It has Rosebud written on it. Kane must have been thinking about his lost childhood at his death. It makes sense now that he was holding a cheap snowglobe. Gee, I guess for all the great things money can buy it can’t buy happiness.
(trombone sound wra wraaa wraaaaaaaa)
Contact:
Jodie Foster’s original research facility gets shut down by the attention-mongering self-aggrandizing astronomer; she sees it happening but isn’t able to do anything to thwart him. She pursues her research anyway after significant delay and gets in touch with ET but no one will take her seriously, and when they do the government rips the project out of her hands and continues to not take her seriously. They have a contest to determine who will go out and meet ET and beforementioned attention-mongering astronomer beats her out using underhanded tactics. Foster is able to do nothing about it but gets adopted by a shady 3rd-party guy with an agenda of his own who manipulates things (including her). Original “let’s go meet ET” project goes down in flames and shady guy finances second attempt, sending Our Heroine. Jodie Foster meets ET but comes back and no one believes it took places and thinks she’s lying. She can’t come up with a single strategy for what she can do about this…
Hey, wait a minute…
Les Misérables: Valjean drags Javert out of the river and they live happily ever after together. gags
Anna Karenina: Anna pushes her husband and Vronski under the train, and strolls back into town, whistling to herself.
Sunset Blvd
Paramount agrees to make Norma’s film and it revives her career.
Seriously. Now I want to read these Bizarro Harry Potter novels.
Ladyhawke: Due to a horrible miscalculation, the eclipse is tomorrow. Isabeau, still in hawk form, flies into the cathedral. The Bishop orders the guards to kill Etienne and Phillipe and excommunicates Father Imperius.
Bambi: A burning branch gets tangled in Bambi’s antlers. Venison, anyone?
Anne of Green Gables: Anne drowns in Barry’s Pond while the girls are playing Idylls of the King. Gilbert Blythe sees her drown but, due to a leak in his own boat, can’t reach her in time to save her. He becomes a haunted man, eventually drinking himself to death in the attic of his family’s house. Diana, Jane, and Ruby are sent to distant relatives where they eventually grow up and marry but are never able to forget that they caused Anne’s death. Matthew and Marilla die of loneliness.
The Dictionary: aard·vark
Pronunciation: 'ärd-"värk
Function: noun
a large burrowing nocturnal ungulate mammal (Orycteropus afer) of sub-Saharan Africa that has a long snout, extensile tongue, powerful claws, large ears, and heavy tail and feeds especially on termites and ants
The Hobbit - Bard’s arrow misses Smaug, and after the dragon incinerates the lake town, he flies back to the Lonely Mountain and makes Dwarf-Kebobs. Bilbo escapes, only to be captured by Gollum, who takes back the ring and makes Bilbo wear dresses and dance little Hobbit Jigs for his amusement.
Star Wars - After her rescue, Leia rewards Luke, Lewinsky style. They get it on again at the rebel base, and Han takes pics of their roll in the hay and posts them on the internet. A distracted Luke misses his shot in the Death Star trench, and the rebel base is soon destroyed. The survivors flee to Hoth.
Empire Strikes Back - Leia has a threesome with Han and Chewie after their escape from the imperial fleet. Luke feels momentarily queasy when he learns Leia is his sister but then warms up to the idea. Cloud City is completely destroyed by the imperial forces, and Luke gives Daddy a hug, but then still escapes because he can’t get Leia out of his head.
Return of the Jedi - Jabba gives it to Leia, slug style. Luke turns to the dark side and helps Vader destroy the rebel fleet. The ewoks are rounded up, killed, stuffed, and sold as collectibles. C3PO is jettisoned into the nearest star because he just won’t shut up. Luke brings Leia back home to be his imperial concubine, and he hangs the frozen Han Solo over his mantle.
Little Women - Father March did not go to war, but instead joined the California Gold Rush (a little late). Despondent, Marmie turns the home into a bordello. Beth dies of syphillis - Jo announces that she’s a lesbian - Amy finds she has a flair for bondage - and Meg gives birth to Laurie’s bastard, who is the sole recipient of the Lawrence fortune. Upon hearing this, Father March comes home to meet his “beloved” grandchild (and take over the fortune), Marmie shoots him dead at the front gate.
To hell with Harry Potter, I want to see the version of Saving Private Ryan where Matt Damon is blown into bloody giblets at the end.
Actually, after having seen Terminator 3, I nominate it for this thread.
Dolores Umbridge becomes obsessed with Lucius Malfoy and stalks him. Hagrid gets beaten to death by giants. The centaurs invade the school, killing both Lucius Malfoy and Umbridge. Filch becomes Headmaster and institutes decapitation for violation of school rules. Harry, Ron, and Hermione are killed in the battle at the Ministry of Magic. The next book stars Luna Lovegood.
**Ferris’ ** ending:
…also works nicely for Charlotte’s Web.
The Bible:
Over the battle of Armageddon the Death Star appears. Bye-bye deities.
Sybil:
Dr. Wilbur slices Sybil’s face off and covering herself with the flesh exclaiming “Silence of the lambs!”
I approve only if Tonks accidently slips into a giant time jar and regresses to a 16 year old. She, Luna, and Ginny become Dumbledore’s Angels.
Dumbledore from a floo flame: Good evening, Angels.
The Girls: Good evening, Dumby.
Moonlighting
David and Maddy get married and have 9 kids in 8 years. Maddy becomes a soccer mom and has no time for things like hair styling and fashion designer labels, while David becomes the perfect mini-van driving committed husband, and never even looks at another woman. Oh yes, and he shaves every day.