My friends and I were actually talking about this recently, and we all agree on one thing. Fan-fiction that does not stay true to the story it's based on sucks.

Now we say that very lightly, because there are times where you can steer away from the storyline the fanfic is based on. Alternate Universes (AUs) can be an exception to this rule. In the situation of an AU characters could be the same but in a completely different universe where the logic of the original story doesn't apply. This can be the case when you turn gems from Steven Universe human, or take a Naruto and make them into knights instead of ninjas. Those stories only adopt the characters, not the world they come from, and can be bent all different kinds of ways.

But as for fanfics in which the story remains in the universe they take place in, such as an x reader or a fanfic between your favorite ship in a show, not sticking to the original lore of that universe you're writing about could make the whole story feel off. The lore needs to stay true to the original creator's world. You can't make a story about Ladybug and Chat Noir and make it so that they hide in the ring or earrings themselves instead of their Kwamis. Because logically, according to the lore, they literally can't do that. It's the same as when a little kid is like "my character can fly because I said so." Without their character actually having any reason that they are able to fly.

You can't write a fan-fiction about a character, and completely ignore all the limitations they have inside of their universe. You can't ignore their backstories, abilities, personality, and you definitely can't ignore their drawbacks. A character without weakness, flaw, or limitations is going to come off boring to your readers because there is no conflict. It is also going to affect your readers since they will be confused when the lore doesn't make sense. They are fans of the same fandom as you, that's why they're reading the fan-fiction. If something in your fan-fiction doesn't line up with the pre-established lore of that universe without a logical reason, it's going to make the reader feel stumped.

Fan-fiction can be great, but if it isn't an AU, there are rules to the pre-established universes that need to be followed for it to make sense. Make sure it stays true to the realm in which it is inspired by. It might not seem like it, but simple errors in the universe's lore can really throw a reader off and make them want to stop reading your story. This is the exact reason fan-fiction can be so difficult to write. You need to be able to work around the limitations the original creator set for their characters. And if it proves too difficult, then maybe an AU in which you are in full control would be a better start for you.

Watch a couple episodes of the fan-fiction you are gonna write, that way you can see where the story is currently at and familiarize yourself with what you can and can not use/do. Make sure when you read your writing back, and make sure that it sounds like something that could happen in the actual show/movie/book. When adding characters to the story that weren't there before (such as x readers), make sure they aren't on a completely different level from the original characters. You still want them to come off as a relatable character and make it seem like they fit into that universe. You don't want OCs to stand out too much. If something doesn't sound right to you, don't risk it. It's possible it doesn't sound right because it doesn't mix with the original plot