For starters, let's talk about paragraph structuring.
Try to make each paragraph have between 3 to 5 lines on average. Too long paragraphs become overwhelming since it fills the entire screen, left and right, up and down, something which in real life books doesn't happen since they have a free blank area at the borders of the page, with the text in the center, making it easier on the eyes.
Always remember to put "." And not a bunch of "," instead. Try to put at least two or three "." In each paragraph. It helps organize it and also the readability.
Most fanfics use "…" to demonstrate speech and '…' to indicate character thoughts (excluding first-person POVs). When we see them switched, it messes with the reader's mind. It is better to just follow the suit, or make it clear which is which at the start of each chapter.
When you are about to make a speech at the start, the middle, or the end of a paragraph and continue the narrative (first or third person), don't separate the speech and narrative through "." or ",". Just put a caps lock at the first letter right after it and in the work behind it use normal punctuation. Example:
(Good) "I like trains." The robot said as I watched the train. "Truly a magnificent train."
(Bad) "I like trains", the robot said as I watched the train "truly a magnificent train."
Don't forget that at the start of each paragraph and right after a "." The first letter of a word MUST be in caps lock.
When making someone yell, make either the use of proper punctuation or the use of caps lock. Example:
(Good) "YOU DID WHAT???" (Good) "You did what?!" (Bad) "You did what???"
Do not overuse the three points "…" at the end of each paragraph. It gets old way quicker than you imagine. You it on special occasions, to indicate suspicion or deep thought or absurd quantities of something or perpetual motion of some kind. If your character has deep thoughts all the time or is sus all the time, then change him. Unless that's just a quirky thing of his. For the main character though, it's unacceptable.
Try to not repeat the same words over and over again. Make a quick google search about the synonyms, antonyms, and expressions related to an X word. Go wild, nothing is stopping you.
When making someone type to another in a phone or other visual interface, try to show it. It gives life to the story and increases immersion. Example:
(Good) [AN0NYM0US/UNKN0WN]: Steve, Im coming for u
(Bad) "Steve, I'm coming for you."
Remember to warn your readers about a change of POV or of a character's point of view in a visible way. Do the same when moving to a different event happen in parallel to another or distant from the previous one.
Another common mistake of fan fiction is points of view done wrong. (writingtipsoasis)
There are three common points of view: first person (where the narrator refers to themselves as "I"), third person limited (where the narrator refers to the protagonist as he/she and only has as much information as the protagonist), and third person omnipresent, where the narrators know everything. To avoid confusing the readers, choose one point of view and stick with it.
Yes, you can switch points of view, however, never do it in the middle of a scene. Let the scene end before you switch to the point of view of another character. Also, if you're writing in the first person, then don't switch to the third person later on in the same chapter. Otherwise, you're just confusing the readers and coming across as inexperienced.
Now, the most important of all: grammar correction programs, browser extensions, and filters, such as Grammarly (a super useful website, browser extension, and more), Gmail (when you create an email to send someone it corrects your words while you type them) and the Microsoft Word correction.
You might even use the three of them or others at the same time, to see if something escaped the correction of one of them. Nobody will complain that's for sure.
A beta reader is also amazing. Create a discord group and ask if someone would like to become your beta reader. Thousands of people will read your fanfic, and one of them accepting is nothing but a matter of time. Even more, if you make a deal about sharing any profits your fanfic does with them.
(quora) Don't randomly include other languages in the text unless it's pertinent to the story and you're fluent. This happens a lot in anime fandoms and it's honestly aggravating. There is no reason to be saying "kawaii" and "Nani" and using Japanese honorifics when you're writing in English. Sometimes I see full-blown sentences in Japanese and the reader is left trying to Google translate stuff that makes no sense. It's frustrating.
This above I partially agree. I've seen some cringe exaggerations, but some that felt fit for the setting, do as you will.
Another thing that's good to say to your readers is your main language. They'll become more understanding of your situation and more tolerant of your mistakes. Dialogue is key.