Think you're missing the mark here, not by much because I don't doubt that futanari are a niche kink but... there are things that work in animation that turn cringe real damn quick when you try to live action them. Not just for porn either, remember the live action DBZ? Either of them?
Xianxia movies exist and do well. Live Action DBZ was just a failure to adapt the property due to a misunderstanding of why people like the original property.
Also tons of anime-concept turned JAV worked really well.
WANZ had an entire series about magical gender bender, where a guy's friends got magically gender-bent into girls, and the "girl" versions didn't lose their libido, and didn't want to just sleep around so it became a harem. That one video turned into a series considering how popular it was. It was based off a doujin and even has a 2D cover.
Tentacle JAV does pretty good.
There's a studio that literally only does Tokusatsu JAV where female power rangers get raped by their enemies, and make a "comeback" with visual effects at the end.
Saying, "they just didn't adapt it right" is I feel a bit of a cop-out answer, since a "concept" transcends all that.
They took female actors, gave them CG penises which they overlayed on top of flesh colored strapons and had the whole working penis shtick.
But like... it's lesbianism. Lesbianism is frighteningly unpopular in JAV. Like scarily so, and futa is just an extension of that. The fact that they didn't see it coming is a surprise. And I guess Japanese otaku are too scared of real people to buy an adaptation of that niche work they like so much.
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To preempt.
Why do I know so much about JAV? Well a Japanese animator I talked to 5 years ago told me that animators try and base their work on reality and use reference pretty often. They told me JAV has a lot of weird content and is useful as reference material, and recommended me a few. I check out the JAV scene from time to time. They get up to weird shit.
Trust me I'm way more fucked up than I let on.
Strange that you like Zell or reverse rape content at all then.
All the fucked up people I meet like normal rape and stay away from Zell, Dieselmine, and other such circles. I mean you don't get
guro, ryona, heavy tentacle play or any of that stuff in femdom works. The tone is usually light in femdom works. Like seriously when someone does even light NTR like Succubus Academia it gets a large negative reaction.
The weird part of femdom is pegging. Girl puts a phallic instrument into a man's ass, which you're not into... and vore? Like those are the only two things... WAIT. Golden showers too... So 3 things I can think of off the top of my head.