We all know and love the story Magic Diapers, where wearing the magical diapers would transform the wearer into whatever age they last thought of until they use the potty. And I have a story cooking up where a young cosplayer girl finds a gem in her new home that she can use to alter reality if she offers it a sacrifice of anything on her person, or even a part of herself. And gets around that by making a reusable diaper with the gem as a a centerpiece on the front of it, and after holding it all day, promptly wets and soils before using it to enact change. However, since it’s using her bodily waste the changes can only be so small, and she works around it by altering reality in tiny ways that build up until society gets to a point where she can wear and use her diapers in public without anyone batting an eye.
But other than age regression/shrinking, and alteration of reality, what sorts of practical application would a magical diaper have? Being fuelled by waste is optional.