The current cover of Cynthia Lord's novel Rules includes a rubber duck floating above a fish. This image references the times when David, the autistic brother of the narrator (Catherine), places toys into the fish tank. One of her rules for her brother, regarding this, is "no toys in the fish tank."
Ideas for a new cover would depict events or ideas similar to that of the current cover. One suggestion would to make the cover using different rules for David. One of Catherine's funnier rules is to chew with one's mouth closed. This would make for a good cover as well.
David has favorite items. One could use one of his favorite items to create a new cover. One of his favorite items is his red umbrella. He also likes the video store. One could play off of the idea of movies and rules and create a cover which shows movies with the titles being some of David's rules.