So the Exhibition Tutorial mentioned there will never be any touching or participation of others with Serena (and possibly Kai in the future?), but what about those of us who are into that? Not NTR, but Swinging. Can it be an opt-in like other content? Hell, you could even make it so that it's not Serena who gets touched, but Yua who does it. Like a scene where Serena flashes her tits, some rando sees it and gets hard, then Yua strokes him off in front of Serena or something. It wouldn't have to be anything crazy/cucky like Serana gets gangbanged and impregnated in public by other dudes while Yua watches (though I would opt-in for that option, too, if it was available, haha).
I was also thinking of a way you could get away with allowing Yua to get pregnant and not have to make too many more assets. Just like how she has magic panties that hide her dick and prevent erections, she could have a magic corset that hides her pregnant stomach that she "has" to wear in order to move around and do anything. That way you could do renders where she takes it off and exposes the pregnant belly, but you wouldn't have to make more assets (I think? Or very few, maybe). Well, I guess if she gives birth, there would be a new child in the game-world, so that asset would be without saying, lol.
Just a thought. I'm not a programmer or game developer, so I don't know how feasible that would be.