An example off the top of my head would be writing a serializer/deserializer pair and explicitly saying they are inverses of each other, as opposed to having to implicitly say that in a round-trip property test. Another idea would be being able to say that two data representations are isomorphic under a given transformation, so I can define a function for one transformation and call it on the other. I don't know how useful this would be in practice, but it seems interesting to me!