What do I need to work on as a writer?

I'm kind of a writer too. I write my own fantasia stories on this journal app I have. That's what I have to write my stuff on. It depends on what you think you really need to write your stuff. Do you know what kind of stuff to write? Do you know the sum or the plot of what you want to write? The setting? The beginning and the end? Just how the entire story is gonna go? The characters and their names? The main character? What all do you need?

Grammatically wise, you are doing a pretty good job, but when it comes to character diction, you could improve. For example, "no offense, but no" starts off really timid(more so in the vein of early Fluttershy) then gets suddenly short and direct(which is mais in tune with modern Fluttershy). It's an odd piece of dialogue that sends mixed signals about Fluttershy as a character. Since you are using the human Fluttershy, I would take advantage of the triple dots and have her stutter once and while("w-well...you see..um..*almost to a whisper* I'm so, so, sorry, b-but I
c-can't...sorry..."). Keep in mind the stuttering thing it's useful for socially awkward or nervous characters as well.

Another example, of bad character diction, would be here: "Too Bad" Celestia being a motherly passive figure, wouldn't be so direct(think mais Yoda(Star Wars) when composição literária Celestia), it'll be mais like this: "A truly essential show would place equal amount of importance on the audience and the performance itself." Watch a bunch of clips of Celestia if you have to, but diction(especially in fanfics) is important when portraying a character.

I also thought you do Trixie's character better as well. Even though she is egotistical and boastful, you have it portrayed in a very flat manner. You have to remember her overdramatic superstar side with it, she isn't simply saying she is better than you, but that she empathizing how extraordinary she is through her talent. This is what you could say is the difference between Rainbowdash's "I'm Awesome" and Trixie's "Tricks up my sleeves". This is why she makes such a big spectacular event in getting something from the vending machine in the first movie. Trixie similarily to Tanaka(from DR2) is a creative composição literária character. To make her work, and make sure they don't get repetitive you need to think of unique ways to stress their personalities. As a tip use boné, cap locks to create different meanings and/or emphasize words(i.e WHAT did you say? What DID you say?).

Also, as a side note, Dangaronpa and Undertale being text and character-driven games are great places to look when doing character writing. I honestly, wouldn't have known how to critique your fanfic if I haven't been such a big fã of both DR and UT. Both do an amazing job at creating an atmosphere around their characters(and character diction is important for that).