I'm glad you like my idea! I've really been wanting to post it up for a while now, but I couldn't get the wording right.
Anyway, you're exactly right about the elements: Water, Fire, Earth, an Air. Now I wanted everyone to be able to do more with them than usual. Like whoever controls Earth can also heal minor wounds, make plants grow, things like that. Someone who has dominion over Air could also change the weather, such as creating a a tornado, which of course they wouldn't be able to do until they fully grasp control of their powers. And as for psychic ability, anything on this
list counts as an ability, like Telepathy, Clairvoyance, et cetera.
I'd like the girl to be sort of innocent and kind, childlike in a way, she doesn't really know how important she is, but as the story progresses she becomes more aware of the task at hand.