It looks like Hyrule Warriors could be the first Zelda game to feature DLC. While this may surprise many, DLC has been a regular part of the Dynasty Warriors series for some time. Based on the series past, it is usually unobtrusive stuff such as new costumes and the occasional new item. There have been a few cases where they added new maps, but that’s not nearly as common as the costumes.
The treasure box set includes some extra goodies such as an official art book, triforce clock, hero’s scarf, treasure chest shaped case with sound, and 3 costume sets. It is noted that the costume sets will also be available as paid DLC, which should give you an idea what to expect from Hyrule Warriors DLC.