What about a specified list of 3-5 least represented heroes with 2-3 different primary material types/themes. That would be restrictive but still give 6-15 different avenues for exploration.
@Tvidotto: Thanks for the reminder. Yes, it is the final. But I overlooked the point of the rules that said the item has to be submitted to the workshop first. I'm part of a team and we wanted to release the item in a collection together with other items and unfortunately the others were not done in time and still are not.…