To expand the description, had an idea for a guy to date a girl for 6 months or so. However long you need for him to be invested in the relationship.  The catch is, in all this time, he's never been to her home. When he finally does, and they inevitably move things to the bedroom, she introduces him to her living toy collection. Dozens of pieces of all shapes and types, formerly human, carefully stored and curated. Dude is dumbstruck, but goes along with her. She Dominates him with her toys. Nothing extreme, not enough to scare him away, but enough to shake his confidence in himself.

The rest could vary from here. He could become her Sub and continue to have various toys used on him. He could realize he wants to transform and become a girl or sexpet or monster or even another toy in her collection. Perhaps he dominates her or even adds her to her own collection. Maybe she even wants that.

How are the transformations done? D-cum? Rune Magic? Artifacts?

Do you have another idea? I'd love to read it.