After you clear Misha and Aurica's Cosmospheres, you can access the special 'Level E' world. These worlds are based on the games Atelier Marie and Atelier Elie, respectively. After you complete the Cosmosphere, Aurica will unlock the 'Marlone' costume, and Misha the 'Elfir' costume, which are based on the Atelier heroines' outfits.

Finally, while not an unlockable costume, there is art of Shurelia in Lita's outfit from Atelier Iris; you can unlock it by getting one of her endings.