Literary Necrophilia

""The Warble, Peddle and Leek Publishing Company proudly announces Romeo and Juliet II--a sweeping saga of lust and passion that begins where the best-selling original left off! The story begins with the discovery that the two lovers didn't really stab themselves hard enough to die, and follows them through their lustful and passionate efforts to escape the clutches of their warring families and find a peaceful life of lust and passion! Now on sale at every drugstore and supermarket in the world.""

- Dave Barry, "Compressed Classics"

This is what happens when you write a sequel to a series that were finished long ago, or continue the work of a dead author without his permission. Not to be confused with literal necrophilia or Literary Mash Ups that involve zombies.

Some authors such as Tolkien and Lewis are famous for avoiding this by virtue of a throng of fanatic fans.

Comic Books

 * The League of Extraordinary Gentlemen is basically a Literary Necrophiliac orgy.

Film

 * Walt Disney (arguably willingly) and his many less than happy subordinates and co-creators.

Literature

 * Jean Rhys's Wide Sargasso Sea relates the plot of Jane Eyre from the POV of Rochester's mad Creole wife. It's unimaginably brilliant.
 * Scarlett, the sequel to Gone with the Wind.
 * And The Wind Done Gone, telling the original story from the point of view of the slave characters.
 * And Rhett Butler's People, from the point of view of...guess. The difference is, this one is authorized, and it adds a new ending.
 * There are a billion and one sequels to Pride and Prejudice. Readers apparently can't stand Lizzie never getting into Darcy's pants.
 * Well, Lizzie and Darcy did get married at the end of the book, so it's strongly implied that they did get into each other's pants. It's more like readers apparently can't stand never seeing the actual act happen on-page. To correct this there has recently been released a "Wild And Wanton Pride & Prejudice" which includes all the dirty naughty bits that readers have been hungering for. Jane Austen's ghost will come to haunt Michelle Pillow yet...
 * For every continuation of the original story, there must be at least ten versions of the original novel told from Mr. Darcy's perspective.
 * Come now, how has no one mentioned Pride and Prejudice and ZOMBIES!
 * Joan Aiken did a continuation of Mansfield Park, featuring Fanny's lively young sister Susan and Edmund's brother Tom, called Mansfield Revisited, as well as a Perspective Flip of Emma called Jane Fairfax.
 * The Time Ships is a sequel to The Time Machine written by Stephen Baxter.
 * Flatland, of all things, has many, including one penned by the original square's granddaughter, Victoria A. Line.
 * The Tripods is a weird double example. The original trilogy is essentially a sequel to The War of the Worlds, written 70 years later. Then 20 years after the conclusion of the original trilogy, John Christopher decided to write a prequel to his old series, which winds up being basically The War of the Worlds all over again.
 * Night Of The Triffids adds extra gunfire and explosions and throws in some Hollywood Evolution as well, yet still manages a pretty fair stab at emulating John Wyndham's style of writing. Not exactly high art, but a decent page-turner.
 * Dune - best summed up in this Penny Arcade strip.
 * The Wonderful Wizard of Oz has 39 official sequels (all together making up "The Famous Forty"), 26 of which were written by other authors after L. Frank Baum's death. The unofficial sequels number in the hundreds - Wicked is just the most famous of them.
 * All Public Domain Characters. This plus the First Law of Resurrection means that Dracula will never stay in the ground.
 * The estate of Arthur Conan Doyle has recently approved one such attempt. In this case, it's by Anthony Horowitz.
 * Of course, given the above point about Public Domain Characters, people were coming up with new adventures for Holmes and Watson long before this. At least one has sardonically parodied this by noting that if Dr. John Watson wrote everything that has been published purporting to be a newly discovered Sherlock Holmes case, then he would have been among the most prolific authors to have ever lived.
 * Another noteworthy example is the Enola Holmes books, which stars Sherlock's irrepressible younger sister, whom he managed to never mention in all his adventures.
 * Many, many religious texts have had "lost texts". Let's just leave it at that.
 * H, a "sequel" of sorts to Wuthering Heights that covers what exactly Heathcliff did during his three years spent away. It eventually turns into a crossover with Jane Eyre, as Heathcliff turns out to be
 * Alice in Wonderland has Alice Through The Needle's Eye by Gilbert Aldair and the revisionist series The Looking Glass Wars.
 * Peter Pan has fallen victim to this countless times. Examples include:
 * The authorized sequel, Peter Pan in Scarlet
 * Peter and the Starcatchers (a reimagined backstory series)
 * The Disney Fairies series
 * Wendy (a Darker and Edgier prequel)
 * Lost Girls (a Hotter and Sexier sequel)
 * Capt. Hook: Adventures of a Notorious Youth (a prequel about Captain Hook's school days)
 * Tigerheart (centered around a Canon Foreigner Lost Boy)
 * Peter Pan and the Only Children (written by Gilbert Aldair, who had previously indulged in this trope with a third Alice in Wonderland book)
 * The Adventures of Tom Sawyer has Becky: A Novel, basically a romantic drama Perspective Flip with feminist leanings, centered around Becky Thatcher as a grown woman. It retcons most of the original book by explaining that Mark Twain was their Unreliable Narrator buddy who idolized Tom and skewed the story for the sake of Rule of Cool. (For instance, Injun Joe was falsely accused, and Becky accompanied the boys on all their adventures. In other words, what really happened was a lot more PC.)
 * And while we're at it, The Adventures of Huckleberry Finn has the Perspective Flip prequels Finn, about Huck's dad, and My Jim, about Jim's wife.
 * Rosencrantz and Guildenstern Are Dead by Tom Stoppard, a perspective flip of Hamlet and a hallmark of Theatre of the Absurd.
 * Grendel by John Gardner goes all the way back to Beowulf and, like the previous example, has philosophical overtones which make it a school staple.
 * Mistress Mashams Repose by T. H. White is a two-and-a-half-centuries-later sequel to the Lilliputian parts of Gullivers Travels.
 * Susan Hill's Mrs. De Winter is a sequel to Daphne Du Maurier's Rebecca.
 * Les Misérables has two sequels....both of which are titled "Cosette".

Live Action TV

 * The Moomins, a Finnish series.
 * Sesame Street (poor cookie monster).