I beat the Kiln. All I gotta say is W. If you make the end of the Kiln have some sort of story reward like You revive from the stone in the first/main level and reunite with the green woman. My view is the main story is like you made it but everyone who makes it there goes into this fever dream which is the kiln, and if you manage to reach the end you can free yourself from turning into one of the others. This could go any way. Good ending would be freeing everyone. Cool ending would be some kind of boss or going against the centipede. Sad ending would be turning also into a green spirit and hug woman at the end of kiln. I know you said its a bonus, but you should consider just adding an extra little ending to make it rewarding.