Fanfiction is new. It is possible to find antecedents for it only by diluting the concept. The Aeneid is not fanfiction; Hamlet is not fanfiction; Paradise Lost is not fanfiction. Not even Sherlockian pastiche is fanfiction. Fanfiction has the form and the ethos of its environment, the fandom.
I should give an example of this kind of distinction. The periodical essay series is distinct from its offspring, the newspaper column. The column is of regular length, usually short; it must maintain a predictable uniformity of tone, subject, and approach; it must have unobjectionable (and therefore unambitious) language; it must be honed to local or party appeal (and therefore perishable). The periodical essay and the column have the same muse, but they are different things.
Why is the Aeneid not a fanfic? The gambit of raising a minor to a main character irresistibly recalls fanfiction. Aeneas is the Blaise Zabini of classical literature. But no one writes a fanfic to prove the power of their language or to glorify their sovran. As the proof of Latin's equality in poetic power with Greek; as the scripture of Rome's world-conquering and law-giving destiny; as a renewal of the Roman spirit after the end of the Republic; Virgil achieved with the Aeneid what fanfiction would never dare to aim at—not because it is new, but because it is fanfiction. The Aeneid mattered to Virgil and Virgil's age, and it is written like it matters. Fanfiction, even the best fanfiction, does not try to matter. It always means less than its model. It is secondary entertainment, something to do between volumes or seasons.
It has been argued, I recall, that Shakespeare was able to dispense with introductory exposition to Hamlet because the story's shape was already known to his audience. The story of Amlethus was old before Shakespeare, and had been dramatized before. But this was not fanfiction. Authors of fanfiction alter and re-invent as tribute and experiment, not judgment. Hamlet is better the story of Amlethus. Hamlet throws Amlethus into shadow, but a fanfic can never overtake its model.
I cannot see that Paradise Lost fulfills any instrumentally religious purpose. Who has Milton converted to Christianity? To Protestantism? No one mines Paradise Lost for sermon texts. Paradise Lost is a proof of the equal literary dignity of Christianity with Paganism, but no one was asking. Those who take Christianity seriously need not take literature seriously. Those who take literature seriously, but not Christianity, will not be moved to reconsider Christianity by its exposition along Homeric lines unless Homer moves them to Zeus-worship. To those who take both literature and Christianity seriously it is a delightful confirmation of their harmony, and of the worth of maintaining a balanced position. But it is after the fact.
What Milton produced was not an apology or a tract, but a statement of his own faith as a learned man, an (idiosyncratically) religious man, and a poet, who believed that these plainly good things went together and did each other good. Fanfiction makes no statements at all. It is as ephemeral as a mood; it is easily found and easily lost; and it always obeys the literary democracy of a fandom. To make statements a writer must be a despot. Nothing abstract forbids such despotism over borrowed characters, but that would not be fanfiction.
Sherlock Holmes is one of the first modern characters. A generation earlier his coolness and calculation would have made him a fine villain; he is modern because he is the hero. Why can't new adventures for Sherlock Holmes be fanfiction? Many are: stories of Holmes time-traveling or displaced through time, or slashed, or crossing over into Yog-Sothothery may be fanfiction. (I think that the first Mythos story I ever encountered was a Lovecraftian Giant Rat of Sumatra.) But The Case of the Man Who was Wanted, or Rathbone's World War II Holmes, or Edith Meiser's radio plays, are not fanfiction: not only because they were written for profit, but also because they were written to stand alone. You could, without prior knowledge of Sherlock Holmes, go to see Terror by Night or tune into Death is a Golden Arrow and come away with a sense of Sherlock Holmes. But fanfiction is written to expect knowledge of the original work. The original may be less a model than a shared vocabulary of allusions (as classical mythology has been to all Western literature).
Fanfiction is not folklore. Fanfiction is democratic; folklore is the people's. The changes of folklore are like the changes of unwritten languages and dialects. No one decrees or enforces them, but they do not express popular consent. They have their own laws which indifferently roll the masses along. Illiterate or isolated individuals cannot be truly creative because, having nothing to compare their stories to, they cannot know that they could have been different, and might be changed. There is an adventitious fanon for every fandom; but there is also a stand against plagiarism between authors of fanfiction. Writers of fanfiction observe the idea of authorship that printing formed.
The phenomenon of fanfiction says something good about the modern world. To me it seems to be rearing a generation of good readers, at once hungry and discriminating.
Most fanfiction is bad; but it is gainfully bad. It is badly constructed, badly thought out or not thought out at all. The characterization is flat or inconsistent. When the plot moves, you can see the strings. Of course. These are first attempts. These are young minds finding their legs, or old minds stretching atrophied limbs.
If they do no more than try, they still gain, and where there is a small gain for some there is gain for all. A mind that has failed in the rigor and endurance composition requires is superior to the untried but confident mind—"I could write, if I could just find the time"—because it respects what it cannot do.
But some succeed by rising steps. They begin to learn the art of writing, along with the practical, writerly side of criticism, not acquired from precepts but burned in by trial and error. This will not multiply masterpieces; they have their own law that keeps them rare. But even an apprentice or journeyman writer becomes, in literature at least, impatient with affectation and intolerant of bullshit.
Fanfiction shows that there is still blood in civilization. As it is a rude, raucous, lawless thing, as it is grotesque and rantipole, it proves that it grows in good soil. The roots it is growing may yet keep that untended soil from washing away.