If they're gonna have guest costumes, they should have guest casting!
A guest costume should reflect the ideal. In Walt's time that probably would have been a suit for a guy. A suit is formal and shows you are taking the park seriously. There's no need for fanny packs. You trust your host, Walt, to provide everything you need at a reasonable price. A female would wear a nice dress, but her guy would still oogle the tour guides in their short skirts. The guest should have a high level of refinement and style and is not easily fooled by cheap merchandise. These guests would fit right in wondering around the unique stores and antique shops of New Orleans Square.
Today, however, the ideal guest costume in the eyes of Disney corp. might be a lanyard covered with overpriced chunks of tin, and free arms to carry lots of merchandise. Instead of the healthly slim look of guests past, today's ideal guest looks ready and able to eat a thousand churros. The guest should give no evidence of a discerning taste, least they detect the hundred times markup on most items.