Futanari Mermaids

Sex on the beach with Sayaka draws an audience: four mermaids, watching from the water, decide they want to be exactly like her. Their wish reaches the sea queen, and they get their futanari bodies granted on the spot.

What follows is a harem problem the protagonist didn't ask for. Four newly-transformed mermaids now want him, and they're not shy about it, which means juggling attention, jealousy, and whatever Sayaka thinks about being the template for all this.

The game plays out as a visual novel: read, choose, watch the cast circle back around to the beach where it started. Story and character reactions carry it more than any branching systems, so the loop is really about seeing who gets their turn and how the group settles into sharing him.