No problem!π You could potentially think of some interesting story for why the otoko no musume is lost in the valley and how he transforms into a futanari girl.
Maybe there's a special mushroom hidden in the valley that turns anyone into a futanari. Or maybe a special accessory/treasure that turns anyone into a futa, when worn. Maybe there's a special futanari genie that can grant anyone's wish to become a futa girl. When the futanari genie has sex with the otoko no musume and cums inside them, they'll transform into a sexy futa girl.π
When she finally transforms into a futa girl, she'll be so eager and let Nene fuck her, using her newly acquired boobs and pussy. Afterward, she can change back into an otoko no musume or futa girl, whenever she wants, via a toggle option.
These are some example of ideas if you wanted to use them or you can make something else up, if you do plan to add the otoko no musume character. She could have 1 event sex scene, when she's an otoko no musume and another, when she becomes a full futanari girl.
I hope to see the futa girl who starts of as an otoko no musume, as it could be really fun to design a cute otoko no musume that turns into a sexy futanariπ₯°.Plus it could potentially lead to an interesting and fun story.