As I understand the thing it does work loke this:
1- We need to expand Dispossessed
2- Artist work on whatsoever concept they like and bring them to the rule team.
3- The rule team makes rules for the piece.
Am I wrong? In my note I have, right now, the Giant stone golem, some cavalry, a drill but i guess there's no point in proposing new units, that's not how they come to be.