That subject is fan fiction. Yes, I can hear you groaning already, and it’s a truism that an awful lot of fan fiction is terrible. Some of it is intentionally so, like infamous Harry Potter fan fiction “My Immortal.” However when most people think of fan fiction, they groan due to Sturgeon’s Law, which roughly states that “ninety percent of everything is crap.”
However, that other ten per cent contains some gold, and many, many published authors started out writing fan fiction. I don’t just mean the inexplicable success of 50 Shades of Grey (which I’ve never read but has quite the negative reputation) which started out as fan fiction set in the Twilight universe. For example, Lois McMaster Bujold’s Vorkosigan Saga started out as Star Trek fan fiction, and the main character, Miles Vorkosigan, was clearly based off of the Klingons, though this has certainly changed as more and more novels have added to the saga and backstory.
Fan fiction is sometimes seen as a cop out or writing with the training wheels still attached, as you’re playing with someone else’s characters and world. I personally don’t see how this is a bad thing. After all, one of the common ways of landing a job as a script writer in TV was to send in a “spec script” which was basically a script for an existing TV show, so showing you have an ear to a character’s voice and mannerisms and how a world works.
It is to an extent an exercise in writing as training. If you’re not great at world building, writing fan fiction might allow you to write a compelling narrative and a memorable original character or two (but please no perfect flawless characters) without having to figure out details of setting. I’ve done it a couple of times (I don’t think any of it is on the internet which is probably just as well, as I find it cringeworthy), once for Buffy the Vampire Slayer fandom and once for Transformers fandom and I think it helped my writing then. It’s a good way of figuring out structure and plots in a fairly tight word count.
And when you stop and think about it, what we now consider classic literature could be considered the antecedent to modern day fan fiction. After all, a lot of elements of the classical Arthurian mythology were added by later writers as the oral tradition was passed down. In the earliest Arthurian tales, Lancelot didn’t exist and Sir Gawain was the greatest knight. So in adding Lancelot, Chretien de Troyes was creating fan fiction canon. Now, Lancelot is definitely an integral part of the mythos and his affair with Guinevere a key part of Arthur’s downfall.
Heck, in one of his works, Plato complained that Aeschylus got the characterization of Achilles and Patrocles wrong compared to Homer’s portrayal. That means fan fiction (and “shippers”) goes at least as far back as 380 BC!