Compton gives good advice to making generators: specific and concrete “artifacts”. Get expertise from someone who makes or knows those artifacts to form a list of characteristics to generate. Create a structure of rules that govern the limits of the generator’s variability and creation guidelines. The usage of Perlin noise and other randomness can be used to introduce parametric change with user input.
It is hard to answer Compton’s question of how to “debug” a generator which isn’t producing the creatures as predicted . To me, it requires technical knowledge of coding and a well made architecture so that it is easy to locate sources of variation in the generator. However, perceptual uniqueness is even harder to identify. This isn’t computer defined and is purely determined by the artist eyeballing the work to see if a human can feel interested in the different productions. For me, a periodic break so I can come back and look at the generated work with a fresh set of eyes helps me determine the perceptual differences in the artifacts produced.