Anime Logic can be Overlooked. Things like-
-Crying unnatural amount of tears
-Joke injuries like Nosebleeds, spitting blood and fainting
If you wanna write a more Laidback and Comedic Fanfic, you could try making the Main Character point it out, joke about the plot holes or break the 4th wall.
THINGS TO KEEP IN MIND:
With a mix of Serious and Realistic Moments to incredibly silly Stunts and Gags, Fanfics will be difficult to write using the exact same World laws.
-To craft New Laws, some silly and some Hyper Realistic details must be either overlooked and forgotten or removed.
They are School Kids, so try to keep it fairly realistic in terms of Intellect and Maturity(this might exclude geniuses like Todoroki, Momo, Bakugou)
-Once again, they are SCHOOL KIDS, don't forget!
-Anything you can possibly imagine can be Quirk.
If you wanna write an OC Fanfic, might want to replace someone from the story to fit your Character in. Here are some of my suggestions:
-Kaminari Denki
-Toru Hagakure
-Mineta Minoru
-Mezo Shoji
-Koji Koda
These Characters are mostly Comic Relief and have very little effect on the Main Story.
(I haven't read the Manga yet so they might be important later on. IF you run intoa similar problem, just say it's AU and forget about it.)