I feel that fanfic has a place for those who are trying to learn how to plot and write. I worked out a plot for an alternate universe when I was in junior high school, but I could not figure out how to translate my alterverse into a format anyone would want to read. Frustrating.
Later, I wrote a couple of Star Trek fanfics. One involved Kirk & Co. finding a sleeper ship full of military telepaths. I wince when I think about it.
Another was a translation of a 1937 US Navy exercise run by Admiral Jacob Reeves, then CINCPAC, into the Trekverse. What Reeves did back in 1937 was, with no warning, send a message to all the ships in Pearl Harbor: "Get underway immediately and rendezvous off Point Loma, California, in 96 hours." In Navy-speak, "immediately" means 'RIGHT NOW AND NO FOOLING.' It was a quiet Sunday. Most of the ship captains and their department heads were ashore with their families or off playing golf or something. But the message was immediate, and the duty officers treated it that way. Battleships steamed out of the harbor with lieutenant commanders in tactical command, while their four-striper skippers jumped up and down on the dock in rage. A couple of destroyers sailed with ensigns fresh out of the Naval Academy on the bridge and no other officers aboard. The Fleet rendezvoused off Point Loma four days later, with most ships at least 25% shorthanded, but ready to fight, which is what Admiral Reeves had wanted to learn. I wanted to set up a situation that put Lieutenant Uhura in command of the Enterprise, which meant I had to get Kirk, Spock, Scotty, and Sulu out of the picture without either killing them or using a doubletalk generator as TAS did to disable all the males aboard in one episode. My pretext was an overhaul combined with long-delayed leave for the crew, with Uhura as the Officer in Tactical Command of the ship when the Point Loma order came down. I was rather proud of the way I solved the problem I'd set myself, and of how Uhura faced off against three Orion pirates and defeated them.
A third involved the Enterprise encountering Harry Mudd again, only he had divorced Stella, remarried, and gone straight. He was running an amusement park planet that was sort of like Westworld (the movie, not the series), only because of his experience with the androids on the planet Mudd, he made very sure to keep the non-sentient droids controlled by a central computer and with emergency kill switches built in. He and his wife owned the whole world, everything. So, when Stella showed up, steaming mad and refusing to recognize the subspace divorce he had filed (hey - if the Rigel 4 dilithium miners can contract valid subspace radio marriages as they did in "Mudd's Women," logically, valid subspace divorces must be equally legal), Harry needs to get away from her, and also his second wife who is understandably peeved with him. The problem is, how do you commit a crime when for practical purposes you own everything? Kirk & Co. had a delightful time watching Harry Mudd desperately trying to commit a crime so they could arrest him.
I wrote a couple of novels that were more for my own edification than anything else. Then, after having kept away from fiction for many years, I got two ideas and wound up writing two Buffy the Vampire Slayer episodes/stories. I submitted them to the folks writing the comic books, and got personally written rejection letters back, saying in effect, "Good writing, but they don't fit our plot lines."
One was inspired by the episode "Lie to Me," where a friend of Buffy's from her old school comes to Sunnydale and makes a deal with Spike to be "brought across" as a vampire if he delivers the Slayer to Spike. My approach was, "What if a new transfer already knows about vampires, is dying of an inoperable brain tumor as Ford was, but decides to take as many vampires as he can with him -- and does not know about the Slayer?" He crosses paths with Buffy, is recruited into the Scooby Gang, and helps Buffy take down the survivors of a vampire motorcycle club (think the Hells Angels with fangs) who he tangled with in Texas as a cadet at the Marine Military Academy there, who followed him to Sunnydale determined to kill him for offing several of their number. As might be expected, he dies gallantly in combat, not saving Buffy. (I went a long way to make sure the story did not turn into a "Yeoman Mary Sue," a bane of fanfic.)
The other had Buffy and the Scoobies hooking up with the Age of Chivalry Re-Creation Organization, a group reenactors skilled in fighting with steel on foot and on horseback. Buffy and Willow are sent by Giles for training he can't provide but people in ACRO the Watchers' Council know of can. It turns out Cordelia has been to ACRO events while on summer vacation and won archery tournaments, though she has gone to great lengths to keep her involvement with them a secret, and is not happy when she runs across the Scoobies at the ACRO event. Oz and Xander go along because Willow is going. Unbeknownst to our heroes, an evil wizard and his apprentice are planning to open a portal between dimensions and loose a bunch of demons on the world. Buffy, whose fighting skills impress the ACRO knights, finds herself commanding a medieval army; Cordelia is possessed by the spirit of Athena, who transforms Oz into werewolf form, but controlled by Willow; and Xander is possessed by the spirit of Saint George, and with the assistance of a teenage ACRO girl rider who fancies him, has to fight a dragon from horseback with lance and sword. Alll comes right in the end, and after they are released by the spirits who possessed them, Xander and Cordelia reconcile. (The evil sorcerer is killed by a demon who felt he and his were betrayed by him.)
All of these were fun to write, and I regarded them as good training. I used characters created by others whose personalities I understood to help me write my own characters to interact with them.
What I learned by writing fanfic gave me the confidence to begin writing romantic erotica. Not stroke stories, but stories with plots where the sex grows out of the situations in the stories and helps advance the plot while thoroughly titillating the reader. I've published them on an online website, and then on Kindle and another e-pub site. It may not be Tom Clancy or Robert A. Heinlein, but the folk who read my stuff enjoy it, and now and again I get a little check for what I have done. My writing won't make me rich, but I derive a great deal of satisfaction from the knowledge that there are people around the world who like what I have written.