Ascended Fanfic

A story that isn't a Fanfic, but it evolved from one. This can overlap with Serial Numbers Filed Off, but it is by no means necessary. The setting of the new work will often be suspiciously similar to the source material for obvious reasons, but the plots of the two stories can come out very different indeed.

On occasion, a fanfic becomes its own legitimate franchise without bothering to cut all its ties to the original, complete with its own fanbase outside fans of the original, with the blessing of the original work's copyright owners. In some cases, some fans may not even realize that it was a derivative work.

Compare the Weird Al Effect, where fans of a derivative work forget the original existed. Also compare Spiritual Licensee, where a work didn't start as a fanfic but the end result is pretty much the same as this. See also Humble Beginnings.

Contrast Ascended Meme and Ascended Fanon, where derivative material is later incorporated into the canon material.

Anime and Manga

 * The Record of Lodoss War franchise began with edited accounts of a Dungeons and Dragons campaign, quickly becoming popular enough to spawn its own game system, novels, manga, multiple OVAs, and a television anime.
 * Nyoron Churuya San started from a doujinshi.

Comic Books

 * Elf Quest wasn't exactly a Wizards fanfic, but the elves' early design owed something to Ralph Bakshi.

Literature
""...it was started on January 8, 2004, about ten days before I sat down to write a different fantasy AU story and suddenly went, you know what, this isn't so much Fanfic anymore, and these aren't Jack and Stephen, ("this is not my beautiful house") and off I went to frolic with dragons.""
 * Young Adult author Cassandra Clare started off as a fanfic author and later used lines from her own fanfics in her original work. Some plots are clearly copied from Pamela Dean.
 * The Vorkosigan Saga
 * At a very early conceptual stage it was Star Trek fanfiction; Beta Colony and Barrayar are reminiscent of the Federation and the Klingon Empire, respectively. Of course, it diverged away and became its own universe almost immediately.
 * A Bujold Fanfic story, Negri's Boys by Charles Finlay, was rewritten with the serial numbers filed off as The Political Officer, and published in The Magazine Of Fantasy And Science Fiction. It was also nominated for the Hugo and Nebula Award.
 * Similarly, this is Naomi Novik's origin story for the Temeraire series:


 * Novik's Sherlock Holmes fic-turned-published-pastiche, Commonplaces, is another example.
 * The Magicians: Grossman wanted to use Narnia whole at first, but due to copyright issues and "it's just more fun that way" he changed Narnia to Fillory.
 * Many have noted the many similarities between C.S. Lewis' The Chronicles of Narnia and Phillip Pullman's His Dark Materials, which the latter admits can be read as a "response" to the former. The plots wind up being very different, however, and perhaps even more notably, Narnia is basically a Christian allegory (though Lewis didn't like to call it that) while Pullman's works are a critique of Organized Religion.
 * Harry Turtledove's Videssos cycle began as Fourth Age Lord of the Rings fanfic.
 * Wicked, towards The Wonderful Wizard of Oz. It's basically an attempt to make the Wicked Witch of the West into a sympathetic character by showing her backstory and everything that she went through before becoming evil.
 * Fifty Shades Of Grey, literally. Used to be a Twilight fanfic named Master of the Universe. (Not in any way related to He-Man.) Similar examples of "pulled to publish" fanfics plague the Twilight fandom: here is the list.
 * Dennis L. McKiernan's Mithgar novels (The Iron Tower, The Silver Call, Hel's Crucible) started off as Lord of the Rings fanfic. Doomsday wanted to publish. Tolkien Estate said "no". Apparently, changing names was enough to avert lawsuit and so The Iron Tower found itself on the market unscathed.

Myth and Legend

 * Lancelot of the Arthurian legends came by way of French storytellers. Percival was an OC. It could even be said that the entire Arthurian Canon is, in its own way, fanfiction.
 * The Harrowing of Hell story has been considered canon for almost as long as Christian teaching has existed. Why "almost"? Because it's actually a story that a bunch of theologians came up with about a century and a half after the events.
 * Older Than Feudalism: The Aeneid, widely hailed as the first work of literature ever and the model for much that came after, was basically a Perspective Flip Fanfic continuation of The Iliad.

Tabletop Games

 * Dungeons and Dragons is something of a strange case. The game borrowed heavily from JRR Tolkien (and a few other fantasy novels, such as Poul Anderson's Three Hearts and Three Lions), as well as various more obscure mythologies, but had many unique elements as well. The franchise owners have ended up splitting the game rules into "trademarked material" and "open source material", and encourage others to develop derivative material based on the open source components. In addition, the owners have also licensed derivative settings that have gone on to become book or video game series enjoyed by readers who aren't necessarily fans of the pencil and paper game, including Dragonlance, Greyhawk, Forgotten Realms, Planescape, Ravenloft and Spelljammer.
 * The Elder Scrolls setting was originally a home-brew Dungeons and Dragons module.
 * Ditto for Malazan Book of the Fallen, which originated as the two creators' home-brew setting.
 * Rhapsody of Fire based epic storylines of their songs on their D&D campaing.
 * Forgotten Realms were born as a setting before voluntarily going under Dungeons and Dragons, though fleshed out mostly after. Whether basic (Greyhawk) rules were more of inspiration or problem is up to Ed Greenwood -- he both constantly kicked them aside and used new edition change for the "Time Of Troubles" overhaul.
 * Renegade Legion was originally set in the Star Wars Expanded Universe.
 * After the Bomb was originally created for Teenage Mutant Ninja Turtles 'verse.
 * The International Super Teams setting for GURPS had its genesis in an unused timeline and history written for a Villains and Vigilantes campaign in which the author was a player.

Video Games

 * Team Fortress Classic is an official sequel to a Quake mod, which was ported to Half Life's engine. As is Team Fortress 2, by extent.
 * Similarly, Unreal Tournament III has the Greed mode from the Titan Pack, (or UT3 Black, depending where you obtained the game) which started as a mod for Unreal Tournament 2004. His creator, Mystikal, is now working for Epic Games.
 * Counter-Strike? It was a Half Life mod. So was Day of Defeat.
 * And Left 4 Dead was originally inspired by a zombie mod for Counter-Strike.
 * Hours before the Doom modification WAD "Evilution" was set to be released, the creators, Team TNT, were emailed by John Romero offering a publishing deal. It ended up as half of Final Doom.
 * Red Orchestra: Ostfront 41-45, Killing Floor and Alien Swarm were all originally mods for Unreal Tournament 2004. The developers of the first founded Tripwire Interactive, who also published the second. The developers of the third were hired by Valve and eventually ported it to the Source engine.
 * Tower Defense began as mods for Starcraft and Warcraft III but later became a standalone genre.
 * Fall From Heaven Age of Ice was included in "Civilization IV: Beyond The Sword."
 * Trouble in Terrorist Town started as a simple game mode for Garry's Mod, but it went on to win the Fretta contest (a contest to see who could win the best gamemode using the new gamemode framework, called Fretta) and grew in popularity so much it now comes with the game.
 * The "Fudge Pack" and "10th Anniversary" releases of Postal 1 and 2 include the Eternal Damnation and A Week in Paradise mods for the second game.

Web Comics

 * Drowtales was originally a Forgotten Realms fanfic.
 * DMFA was originally set in the Furcadia online chat-game-social-roleplaying...thing. So that would technically make it a fanfic. Early strips have a lot of reference to Roleplaying-terms, internet fads, ect.
 * Freighter Tails was originally called "Haul Trek", but the authors remade it, removing all the Star Trek elements in the process.
 * Whatever it might have become, it's fairly obvious from the first strip that Looking for Group was originally a World of Warcraft comic. Things went full circle when the backers suggested a video game was made.
 * Awkward Zombie had a strip about turning itself into an Ascended Fanfic, but it turned out to be onetime gag and the strip went back to using Smash Bros. characters.
 * The author pulls strip ideas from the forums pretty frequently, and even has a thread for the purpose.
 * Arthur, King of Time and Space is based on King Arthur In Time And Space, which in turn was based on Star Trek/Doctor Who crossover fanfic cartoons with the proper names search-and-replaced (Arthur, Lancelot and Guenevere replacing Kirk, Spock and McCoy; Merlin and Nimue subbing for the Doctor and various companions).

Western Animation

 * The Gender Flip episode of Adventure Time was based on What If comics created by the show's character designer, which was in turn based on the Rule 63 images on 4chan.