I've never really been able to get into fanfiction, for some reason, even about stories I really enjoy and want more of. Maybe I just haven't read the right fanfiction yet. Either way, I've always been more interested in writing my own stories than expanding someone else's. I take plenty of inspiration from what I read and watch, but it's never felt right to write something I didn't come up with myself.
You realize every Single Fantasy Element was already invented by someone else, and you are just gathering countless ingredients and throwing them together with new names? Magic, Knights, Academy, Guilds, Monsters. You just come up with new names for the same things. Fanfiction borrows a specific variation of those elements with an established lore. Instead of mixing them together in an original story, you take what's there as a base and expand it. Good fanfiction authors do not simply rearrange but expand the world, like starting the story before the original, expanding or creating completely new characters, places, and stuff like that. In a sense, fanfiction is harder to write because you have to deeply analyze and then obey the established lore. You are not inserting just a single character in the story, you merely borrow the setting.
I bet if you write an original story, I would be able to come up with dozens of things you didn't come up with yourself. You are riding on the shoulders of giants and you are so small that you can't even spot and recognize them, despite them carrying your entire story silently.
I've never understood the appeal of fanfiction. Writing is like having a magic toybox that can create literally anything you can imagine. Fanfiction is refusing to play with your own magic toybox, and instead going next door to take toys from your neighbor's magic toybox.
Why play cardgames like Poker? Come up with your own games and come up with a new set of cards while you are at it.
Why cook according to cookbooks or what your mom told you, come up with your own recipe.
Why use Excel? Use pen and paper or come up with your own System.
Fanfic is based on supply and demand and takes something beloved that works and expands upon it. Most of the popular franchises are world-famous for a reason and beloved by many. People want more. You could create your own story, which will most likely be inspired by an amalgam of different stories spliced together according to your own taste anyway and won't be as unique as you might think. Only 1 out of tens of thousands of stories will be unique and add something truly new to the Genre. All others are just barely recognizable unofficial fanfics.