Damn am I limited to one
Jk, most of my disliked hcs are character-specific more than ship-specific, but here's a ship one (yes one, but I took my cahnce to go off about it):

Them getting rid of the nickname "Mob".
Leaving "shishou" behind at some point makes sense, sure, but I've been with my partner for over a decade & if she suddenly decided to ""romantically"" call me by my first name by default, I think I'd strangle her a little. (romantically) The first nickname she ever called me by ("danna", which ironically can also translate to master, lol) was last in unironic use yesterday & that one is much more cringe than "Mob".

It's not like I hate all use of "Shigeo" by Reigen, I don't. I can also do with the use of it being romanticized, even though I personally prefer romanticizing the persistence of "Mob" instead, because that just makes more sense to me from both personal POV as well as from how we see the nickname used in canon.
Even my female versions of Shigeo whether cis/trans are named to make "Mob" persist lmao.

What bothers me is the notion of "Mob" being something negative, which usually is either explicit or implicit whenever the first name vs nickname thing comes up. Oftentimes one or both of them starts disliking it, treating it as something with negative assciations, or a "kid thing" that is a last remnant of a past they want/need to get rid of in order to pursue their mature relationship.
I guess in summary it feels like treating a continous part of their canon with disrespect to me & I simply can't get behind portraying something like that as negative in retrospect. Canon is why we all love them after all isn't it??

Anyway, disclaimer - I'm fully aware it's not that serious! I know for most ppl it's just a "that would be hot" thing, half of the time or more it's literally a "say my name, my real name" while they fuck situation & ppl always have my blessings for any of that, I am not immune to picking kink over canon either, etc.
But you see, I am simply genuinely ill about them, so it personally peeves me. Sorry about that.