New Headcanon: Aqua develops an obsession with fashion as a coping mechanism. After Sora rescues her from the Realm of Darkness, her first order of business is to make Sora some new fabulous-looking clothes, which is how he gets the KH3 outfit.
But where did she get the material and equipment to pursue her new passion in the realm of darkness? I mean I guess there's the town from the castle of dreams but would she really be ok with robbing all those people? Fun idea though, it would be pretty funny if it happens.