I don't even know what's considered niche anymore, honestly, but, here's a few that I'm interested in and a couple that may count as uncommon:
consensual_tentacales/consentacles used to be more of a thing, because there was a point when so much tentacle hentai was pretty kinda brutal rape. Not that weird now
rule34.xxx/index.php?page=post&s=view&id=2902603futa urethral_penetration, especially when the smaller cock is the "dom": This one is still pretty weird and a lot of it has too much anatomy focus/veins or ridiculous proportions for me. The concept is hot though
rule34.xxx/index.php?page=post&s=view&id=9611756rule34.xxx/index.php?page=post&s=view&id=9553885women with a little hair pubic_hair, hairy_anus: it's just sort of a nice break from the "pristine" girls in porn (which I can enjoy too). Sometimes a step further if they look a little unkempt and nerdy
rule34.xxx/index.php?page=post&s=view&id=9732430rule34.xxx/index.php?page=post&s=view&id=9892565cuckqueans who are REALLY into wanting to see their partner fuck other girls. Apparently people love to draw Kim Possible this way lol.
rule34.xxx/index.php?page=post&s=view&id=3103694thinking_with_portals this has quite a few pretty creative pics I've seen
rule34.xxx/index.php?page=post&s=view&id=4158312rule34.xxx/index.php?page=post&s=view&id=9054508all_the_way_through I used to like this one a lot, especially if it goes up through one person and into the mouth of another.
rule34.xxx/index.php?page=post&s=view&id=5784557rule34.xxx/index.php?page=post&s=view&id=8975532Thank you for coming to my long winded presentation on the finer points of weird porn. There are snacks in the lobby for anyone left