A world is full of people who can master only one fundamental element. A person is born who has the ability to master all "fundamental elements" but that person is born into a group of people who master one element. When that person dies, he/she is born again to another group of people who master a different element. The person grows up and learns, again, to master all elements, and the Avatar line continues.
That's multiculturalism with a heavy dose of Earth worshiping.
I LOVED the anime series and look forward to seeing the movie.