The Superior Elementals
Every thousand years, two Elemental Princesses become Superior Elementals at the age of 14. They could be from any of the four Element Kingdoms - fire, water, earth, or air.
Sage, Princess of the Fire Kingdom, is turning 14. She reads the legend of the Superior Elementals hoping that she'll become one. She's tired of her boring princess life and wants a change.
When Sage does become one, she realizes that being a Superior Elemental is an extreme task. Something huge uncovers. Old enemies return. Sage and Pearl, the Princess (and Superior Elemental) of the Water Kingdom, must keep the kingdoms together and save the elements.
I give credit to Funomania101 who gave me some awesome ideas for the story!