A theory I remember seeing a while back is that every big fandom produces a weirdly-specific kink structure that recurs over and over in fics for that fandom; usually you can see the connection between work and kink if you squint, but it’s not always obvious. If the fandom gets big enough and the kink isn’t too obscure, this spreads beyond the original fandom.

BDSM AUs originate from Stargate: Atlantis (probably from the military elements); ABO from Teen Wolf (okay sometimes it’s obvious); Homestuck produced kismesis and the other quadrant romances (unsurprisingly, for a work that paints the medium so hard, it’s literal and on-screen). I don’t remember Buffy’s legacy but I’m pretty sure it exists; same for Supernatural. Star Trek’s legacy might be “literally all slash fic” - obviously it’s going to be something drawn from Kirk/Spock but I am not a Fandom Old so I don’t know whether it was more specific. It’s presumably not always kinks per se, either - Slash and Soulmate AU are borderline, and Coffee Shop AU is clearly not a kink, but I’d bet soulmates and coffee shops have a similar origin story to the others.

Notable examples that didn’t spread beyond the original fandom, well, the one that comes to mind right now is Oncest. There’s plenty of less cursed examples but none of them comes to mind right now.

Anyway, maybe all you need to do is expand your horizons of what fic you’re looking at. In some obscure fandom you’ve never heard of, I’m certain there’s a weird new porn with worldbuilding premise waiting to be explored.