She can either be darkness[the org. weilds twilight not darkness], Sun, Twilight, Shadows, i doubt she will be light. Light was already used in Roxas' case and not all keyblade weilders are light.
Xemnas - Nothingness
Xigbar - Space
Xaldin - Wind
Vexen - Ice
Lexeaus - Earth
Zexion - Illusion...