I'm not big on fan fiction. I get the appeal to write for one's self a story that might expand a particular universe for you, or maybe share it with friends. But my wife sometimes gets on the various fan fic websites, and I have read through some of them, and honestly, some of these folks kinda scare me. Especially when it comes to the erotic fiction, or worse, the slash fiction (for those uninitiated, that's where you take heterosexual males in a story and make them gay - not homophobic, just saying don't fetishize sexuality, unless it's girls in school uniforms).
There was a video I saw once where a fan fic writer was ranting about how she would never read anything by Anne Rice because she doesn't allow fan fic of her work. She was afraid she might really like the stories and be tempted to write it, so she was staying away from it. She then went on a rant about how writers don't actually own their characters once they release a book, it's the property of the fans, so to ban fan fic was stupid. First, no, when you're a creator, you own the characters. You created them, and they bend to your will, not someone else's. As an aspiring writer, I would also be pissed if a bunch of people took characters I created and changed them in ways I didn't intend. Second, not reading someone's work because you are afraid you would eventually write fan fic seems rather juvenile. Like, oh, I don't know, don't write the fan fic. Seems pretty straight forward to me. She also made the argument that writing fan fic can help your writing skills and make you better. Sure, I guess. You know what else makes you a better writer? Creating your own characters, situations, and environments, and then writing them. I just remember getting to the end of her video and thinking that this young lady really needed to either grow up or find professional help.
I mean, yeah, I've sat with friends or on my own and thought out scenarios for comic book characters or movie/TV franchise characters, but it's usually in a "what if" situation. The fan fic online community seems too seriously invested in their own work, and at times, they and the shippers have actually tried to affect franchises with petitions and movements to get their visions realized by showrunners and authors. Which to me feels slightly entitled, but I guess that's how the modern world works.