I read fanfics a lot, and I'm really picky on what I read.
First, the grammar and spelling (this one is obvious). As a reader, it is frustrating to see stories that are written poorly. If the story is good, but badly written and unreadable, then it meant nothing.
Second, do not re-hash the stories. It feels like a waste of time reading a fanfic where the OCs changes nothing in the canon. Like, Why are they even there? What's the point of reading this? There should be a story plot (your own). I'm saying this because there's a lot of stories out there that are just plain re-hashing of the canon, with one added character. Like the first one, this should be obvious right? There has to be a substance to your work. A conflict. Give the readers a reason to keep on reading. Since it's fanfiction, readers won't stay if they already know what's going to happen.
Third, the OCs. The attitude, the personality of the OC should be consistent. If they're broody and aloof, they shouldn't go out of their way to greet another character because, oh I don't know? because the author wants to? Unless they're having a character development, characters should not be ooc. Take time to study the characters. Learn what makes them tick, what they likes, what they do on their free time, their favorite food, favorite colors, how they dress, their backstories, what kind of people do they associate with. In short, study the characters first (including your own OC)
Fourth, avoid a mary sue and gary stu character/s.
Fifth, stop putting scenes that are not related to the story. Readers don't need to know the bathroom routine of the OC. We don't need that! Write stuffs that can actually impact the story. Bonding moments (strengthening of relationships), training, also don't hesitate to put the OCs being frustrated and angry and wanting to destroy the world, of course within reason.
uggh, I don't know what else to say, or rather, I don't know how to say it. Just make it good, ask for critics, ask for things that you should improve on. Writing fanfics is just like writing your own story (duh). The only difference is that there are already characters and a story plot for you to use.
Oh, also try reading fanfics. There's a lot of great BNHA fanfics out there. Observe and analyze their work. Our teacher always tells us to Steal like an artist. It's not straight up copying and stealing, but more like observing one's work and applying what you observed and making it your own.