I write a lot on ao3, I have some tips!! (And remember these are tips, not guidelines. All that really matters is that you're having fun when you're writing!)
1. Plan out your ending. If your goal is to write the fic to completion, then always know how you're going to end it. It can be a concept or an entirely fleshed out planned scene, so long as you have your ending in mind, it's a lot easier to write towards that goal.
2. Remember you're writing from a character's point of view! It's up to you whether you're doing first person or third person, but remember which character you're writing as. The language you use for descriptors, dialogue tags, everything counts a lot towards pulling the reader into the story. For example, Bonnie is but a child, and might not have the words to describe things as eloquently as a character like Odile would. It's a writing trick that can take time to pin down, but I've found it adds a lot to characterization!
3. this one is just me being a hardass about writing in general but REMEMBER: Names of any kind are capitalized! This includes names for buildings, or sometimes events. The Universe in ISAT is a personalized being. The House is a named location. Dormont, Vaugarde, Mwudu, Ka Bue, are all going to have their first letter capitalized!
3a. Words with multiple variations can be hard to nail. For example, I see you made an error with your "there", which I. Completely understand for the record. that words variations were ragebaiting me until i was like 14. The easiest way to remember the differences is that "Their" is used to refer to a person. "Their watch", "their herd". It's a possessive form of "there", which is a bunch of grammer terminology WE DONT CARE ABOUT!!!! "They're" is a compound form of "they are", so always remember that the "'re" in that word is a stand in for "are". Went it comes to "aren't", you'd want to use "there", which is like. the noun form. i just use it for whatever "their" or "they're" doesnt cover LOL
3b. i am on my hands and knees begging you to remember to use paragraph breaks. please remember to use paragraph breaks i will do anything. A common mistake I see ie that people will cram entire conversations into one paragraph which is......not correct.... A new character speaking means a new paragraph. Please. Please
4. In relation to Bonnie– or any character who uses they/them pronouns– "Themself" is a valid word. Just so you know ;3
5. Have scenes you're excited to get to! A big motivator for writing for me is to have scenes Im excited to write out. It can also help to understand what needs to happen for you to arrive at those scenes. You don't need a meticulous scene-by-scene timeline of whats going to happen, but for me, some basic ideas of whats gonna happen always helps :P That way you're not totally aimless when writing
6. When you're writing, try to remember that the other characters are experiencing this too. These other characters have their own thoughts, feelings, and ideas about what's going on. It's going to get REALLY repetitive seeing as we're in a time loop, but that's a story direction that is up to you. My best advice is to try not to disregard a character in favor of another.
That's all that comes to mind in one sitting, I hope I didn't come off as too. Rude about it (´゚ω゚`) I don't know how you write so I just gave whatever I could think of. Have fun with your fic!! I'm really invested in this idea and I'm surprised more people haven't approached it, so I'm excited to see what you do with it (*^▽^*)
(PS. Is Loop still gonna be youknowwho in this instance? :3c)