I'll start off by saying everyone starts somewhere, and I understand that not everyone has acquaintances or the abilities to do certain things. (But even then you can put forth an effort.) Anyways-
1. ,"Oh My Arceus!"- Oh my god or oh my gosh is just fine. Please don't use this in dialogue, silly or seriously. It makes me grit my teeth in cringe.
2. Default Battle HUD. Default Move Animations- This might sounds hoighty-toighty and picky-wicky but if you're not even going to put an ounce of effort into the optics of the majority of your gameplay...well-
3. Anime Tropes/Anime Characters- I know that a lot of people for some reason like demure, sweet, lobotomy-stitched cutesy girls who only care about happiness, friendship, and putting their subservient kindness above all else but please- Let's try to have characters exhibit a more complex femininity. Even if you don't understand women. Also, I don't want to see Attack on Titan or Sword Art Online characters anywhere near me, especially in a fangame.
4. Kanto Starters- I promise someone will still play your game even if you have something else to pick besides the unholy trinity.
5. Maymays- Please don't use memes or reference popular meme figureheads. Please. PLEASE! It is single-handedly the most distracting thing I have ever encountered. This includes references to online comics about, "EYUP! THIS POKAMANS MECHANIC SHURE DON'T MAKE NO SENSE! LOL!!"
If you're tempted to poke fun at a certain mechanic either change it or leave the joke out.
Just do your best. (ALSO! I have never seen a YouTuber in a fangame but that sounds insane. I think I'd have to stop playing after that.
Markiplier has a nice body but he's not funny in his videos so how is he going to enhance your game?)