What are REALLY uncommon kinks that have their niche in fandoms?

By uncommon I mean stuff that mainstream but otherwise sane fanfic readers (so, not antis) would be outraged to hear about. Hell, even some more seasoned kinksters would go “uh oh?”

For example:

Tier 1 (yawn):

dom/sub - mainstream as hell, our moms read published romance like this on the bus home from work

Tier 2 (eh, not that outrageous);

pissing - you’d usually not hear people in mainstream fanfic circles talk about it, let alone see it in published romance, but it’s also not that crazy of a kink with a big fandom (haha?) irl and otherwise

Tier 3 (ok, quite a few people are shaking in their boots):

sex at childbirth - you can imagine why some people are into that but the mainstream fanfic community is shaking and crying

Tier 4 (probably offending sensibilities of most people)

wound fucking - uh oh, we’re in it now!

Tier 5

No comment, I have no examples. Maybe you do!

I’m mostly looking for examples of Tier 3 to 5 with no judgment. All of the examples I gave are just that, examples, based on what I’ve actually learned here and what I subjectively would rate in the aforementioned tiers.

I find that the fanfiction world is full of interesting kinks one has never heard about and I love reading about it, so please, give me examples of the most obscure kinks that somehow still have stable readership!