Said of a Fan Fic that attempts to emulate the tone, atmosphere and style of its inspiration as closely as possible. The goal of the Original Flavour fic is to seem as though the original production team could have thought it up. Even if it heads off into previously unknown and unexplored territory for that property, the story will emphasize the use of existing gimmicks and devices rather that introducing totally foreign ones.
If you are writing an Original Flavour fic, you don't introduce or kill core characters, revise continuity, or do anything that is non-canon beyond the events of your story. Original Flavor is exactly like writing a spec script you could submit to the showrunner, but not necessarily in script form.
Note that some fandoms, especially those with looser story arcs, are easier to write in Original Flavour than others. It's also more popular for works with a distinctive style of writing and/or dialogue, as this is a particularly easy element to imitate.
Original Flavour has been the direct polar opposite of most other Fan Fic types at one time or another. Because of the widespread prevalence of labyrinthine Fix Fic, our old friend Mary Sue, and florid Shipping, Original Flavor can be incredibly rare. Fun is also found in a self-imposed challenge to see how far a writer can go out of the lines without technically deviating from "canon", without becoming an overt Elseworld.
For when this treatment is done for individual characters rather than entire stories, see Sailor Earth. For fanfics that take this up a notch, see Script Fic.
Neon Genesis Evangelion's RE-TAKE has the same sort of occasionally Mind Screw-ey plot as the original Evangelion, the art is as good or BETTER than the canon source, the characters are all perfect, and the eventually happy ending is completely believable, taking into account the characters and the situation. Asuka fans consider it to be the true continuation and ending of End of Evangelion, while others feel conflicted about the plot direction it takes, particularly during the later stages of the story. Original Flavour may not always be a good thing.
Online Fan Fic pioneer Ryan Mathews once participated in a hoax wherein an unfinished Dirty Pair fanfiction was released on Usenet as the purported "transcript" of a lost OVA. Apparently, it was Original Flavour enough for those who read it — the record shows some of the readers engaged in several weeks of frantic research.
The longer the Super Mario Worldromhacking scene sticks around, the more its participants rely on different graphics, new music, and unusual game mechanics in order to make their creations stand out from the pack. Return to Dinosaur Land, on the other hand, is a hack consisting almost entirely of levels that could have been in the original game. These sort of hacks are usually referred to as "vanilla" hacks by other members of the scene.
Though it starts out roughly, The Clone Wars: Commando's Dirge winds up having several readers proclaiming it not only tastes just like Karen Traviss' Republic Commando Series (without all the Jedi-bashing, too), it seamlessly brushes against a chapter of Triple Zero, without changing the scene at all.
Astrokath has a particularly good take on the events shortly after Dragonsdawn in Dragon Days.
Team 8, a fanfic for Naruto, really catches the flavor and feel of the show while not sacrificing originality or quality.
Neutronium Dragon's Sojourn would make a dark (and relatively short) installment of Quest for Glory, but as a story, it would fit right in — out-of-left-field goofy moments included.
Todd Smitts' Cold Casefanfics, written as conventional episodes of the series down to the use of music of the period in the flashbacks.
Smitts and others also began to write a fictional 8th season after the series was cancelled, picking some of the lines left loose at the end and even proposing actors for the new roles. You can read it here.
So long as you ignore most of the "special chapters" (about the backstories of the various Alternate Universe characters), which are drawn by different artists and tend to look a little... off. For a good example, look at the disturbing facial expressions in the Bojack special, or the general awfulness (and somewhat excessive gore, compared to canon Dragon Ball or the main comic) of the Cell special.
The Axis Powers Hetalia fic An Incident and Jerusalem/Increments of Jerusalem play out (for the most part) like a classic Sherlock Holmes story, going so far as to capture Sir Arthur C. Doyle's narrative style. This stays consistent all throughout, even as Arthur casually outwits Holmes.
The Joker Blogs. Barring the Mythology Gags, everything is as Original Flavor to The Dark Knight Saga as can be. This is multiplied to the nth degree considering the Joker's characterization, and his performance is so good that a lot of commenters jokingly(?) think that Heath Ledger has come back from the dead.
This author seems to have a taste for Original Flavour. Besides for a few quirks, most of the stories are obvious (but apparently highly effective) attempts at writing actual episodes.
This Pendent Heart is a Princess Tutu fanfic stunningly similar to the tone of the original show, even despite being overtly shippy at times (it helps that the anime is, after all, a shojo work). It also ties together and expands on a few plot threads, while continuing to develop the characters.
There are many, manyHomestuck-derivative fanworks. However, probably the most Original Flavor of them all is Housetrapped, which carefully copies the art style of the original with different characters, and which is being used as the basis for a fan-game.
The Elemental Chess Trilogy, a series of Fullmetal Alchemist fanfics, was intended to be this. It apparently succeeded, since a few reviewers questioned whether the author was really Hiromu Arakawa writing under a fake name.
Meet Nadeshiko, a Shugo Chara! fanfic. It starts out as a generic rimahiko fanfic, but as the story progresses, it becomes a great sequel to the series.
Between Minds by 3theCaptain is one for the Half Life and Portal universe. "If all goes according to plan, this story will not interfere at all with the canon material."
While a number of Pokémon hacks shoot for ambitious story telling, changes to the overall badge collecting formula, or otherwise veer off the main series's very narrow design concept, the Emerald and FireRed hacks Altair, Sirius, and Vega are designed to mirror the main games in every way, from the stat spreads of their new Pokémon down to the style of music, trainers, and towns. About the only thing that's out of place from the typical games is the last of the three hack's extreme difficulty, and their choice of an eighth Gym Leader.
The Ben 10HeroHighSeries pretty much reflects the first episode but then goes off in an entirely new story line with strong character development and stories. If the reviews are to be believed some readers consider these stories better than the actual show. Also the author fully believes the team that works on Ben 10 stole some of his work, as his character Ren looks a lot like and fulfills the same role as girlfriend (which includes similar hair cut, body build, and even Japanese appearance) to Ben as Julie in Ben 10 Alien Force years before the show came on.
Hitman Miami is an unusual example, as it is a text fanfiction of a video game and attemps to convey the feeling of actually playing a Hitman game. It is written like a walkthrough, with each chapter being a different mission, the plot being advanced in "cutscenes".