oooooh this is really interesting bc i think it varies depending on whether i'm writing or reading, and this could get really long, but:

there are certain tropes/themes i refuse to write, just bc i'm not comfortable writing them or just entirely disinterested. i feel like these might be a bit controversial, but to each their own, ya know?
1. established relationship: there are ways this can be done right, don't get me wrong, but it's one of the first tags i filter out when i first start reading for a fandom. if i've finished everything in a ship i like, i end up coming back to these fics, but i prefer a good slow burn with all the yearning and pining of people who aren't together. similar reasons for why i don't write it: i just like writing about the yearning and pining, for one, but i also like writing relationships where the characters are en media res within their own storylines, but the relationship is just about to start.
2. omegaverse/mpreg: i'm grouping these together bc they usually tend to be grouped together within fics. i've read a few in the past out of curiosity, but the concept's not my favorite. it veers into uncomfortably weird territory and i would rather not put myself through that for a hobby.
3. mcd: i actually can't say writing mcd is off the table bc i do like writing angst, even if i haven't committed to it as much lately. there's so many terrible things happening in the world rn, i just think having light fics for people to read rn could be nice. that being said, it's obviously angsty to read lol so i guess this isn't truly a non-negotiable.

as for when i'm writing fics, there's a process to it in which the plot dictates the ship that i end up writing. i think gyuhancheol are a perfect example to use bc they are basically all i write rn so:
1. seungcheol has to be a younger brother. this is so so so crucial to his characterization for me. similarly, mingyu and jeonghan have to be older siblings, but there's a little more flexibility if i want to make another member a younger sibling for the plot. the only exception where cheol would be the older brother would be if vernon was his younger sibling, but even then, i think his characterization shines best when it rings true.
2. if it's a rivals plot: it has to be gyuhan.
3. if it's a childhood friends type plot: it has to be gyucheol.
4. jeongcheol are harder to pin down bc there's a specific image of them within the fandom and i sometimes worry that i lose my own vision for them within this other one. i think my doomsday neighbors encapsulates the dynamic i envision them having best though.
5. fitting the three of them together is a lot like a puzzle, but i picture there being a red herring and an anchor. initially, one of them has to shine and the plot as to appear to revolve around them, only for the final act to center the story around the anchor, navigating a firm balance between all three. they're v three body problem, what can i say?

then again, a good enough plot would let me throw the rules out the window to write it, so that brings me to my final point:

truly, i think the biggest non-negotiable i have is that i won't choose the characters for a plot just to write a pairing, i want the plot and setting to choose the characters to best convey the narrative. a lot of my everyday writing is academic/scientific and i sometimes worry that my fics fall flat bc of it, but the ultimate goal is to evoke emotion within a reader and to tell a specific story.