that post about the lack of fic in opm fandom made me remember this fuckin. essay i wrote like a year ago for a reddit comment that i then shamefully deleted cos it was so entirely irrelevant to the original post. but i saved it so. here it is. a fanthropological analysis on the state of fic in opm fandom:
One Punch Man is not a small fandom by any means, but it definitely seems that most fans are of the 'dudebro who likes to powerscale' variety, and I'm continually surprised by how small the shipping and fanfic writing side of the fandom is in comparison. Like, this is one of the most popular well known anime series in the world, I'm pretty sure even the most casual of anime fans has to have at least heard of it. But it only has 5k fics on AO3, which isn't exactly small, but considering its popularity and in comparison to other popular battle action "shounen" manga (technically OPM is seinen but no one remembers that so), it is bafflingly tiny. I've thought about it a lot, and I think it comes down to multiple related reasons, both internal to the story that limit the likelihood of a fan to write fics for it, and external, lowering the number of ppl who would want to write fics for it that become/stay fans in the first place.
Let's start with internal reasons. The cast of characters is huge, made up of mostly men (a lot of whom are young + attractive). Which seems like it should be a definite boon to M/M shipping--the backbone of a large number of fandoms! But I think there's a difference in that with so many characters, screentime/focus is really spread out across them, and I'd find it hard to really tell you who counts as a "main character". On top of that, there's not a whole lot of interaction between them, they all seem much more isolated. I think this is especially the case in the biggest arc that took 7 years to complete. Even the biggest ship/pair had a ~3 year gap in the manga where they didn't appear together at all.
But there are definitely still lots of ships with potential and interactions to fuel shipping (although I still think less than comparable media), so that's not the whole story. Which brings me to my next point about external factors. Most of those ships are not in the early parts of OPM, and you're probably gonna have to read through the manga and also the webcomic to get to them. Most people have not done this, and have probably only watched season 1 of the anime, considering that was the peak of its popularity. And those early parts are much more comedic, so I think the impression most ppl have of OPM is that it's primarily a gag anime/manga. Which is still true, but the story has definitely gotten more complex and serious. So I think a lot of people who might want to write fic for OPM probably don’t get into it because they don't expect it to have much potential in that regard.
I think the release schedule of the anime also plays a huge role, with the anime taking 4 years to come out with a (widely considered) disappointing second season, and with a third season trailer having been released after like 5 years. It’s probably not an understatement that most anime/manga fandoms are much more focused on the anime side of things, and not everyone who enjoys an anime is going to then get into the manga.
And let's not forget the manga and webcomic release schedule, with the webcomic releasing basically solely according to ONE's whims, and the manga constantly going through redraws. Which I'm not necessarily complaining about since we get all this lovely content for free! But it does pose another hurdle for new potential fans, and I imagine there are quite a few who simply didn't want to put in that commitment over time, or honestly just got confused about what was going on.
OPM as a whole has lost a lot of relevance, it seems not many people are getting into it right now and many fans who did write fic for it at the peak of its popularity have probably left the fandom in search of greener pastures.
This is all just speculation though. I wasn't actually in the fandom when it was popular, only got into it a few years ago, so my theories on its decline aren't super backed up. I hope it gets popular again, having said all that I think there's so much fandom potential that isn't being tapped into. Like others have said, a lot of fandom/fic is based on filling in gaps and aspects that weren't explored, and there's plenty to explore in OPM.