As an addition to custom levels, user-created custom shapes (In addition to the Cube, pipe, ect..). I could see it being trivial to apply the texture/logic of grabbable, metal, bouncy, icy, and glass to a custom mesh.

...Though you'd have to have a check/error message for if the custom shape is a enclosed hull. And I hear unity's collision check doesn't play well with non-convex hulls? So there'd have to be a check for that too? But the vertices of a mesh should be a json-able list that you could throw right in your already existing save file format?