Avatar: the 5th element

I thought about this one when I was thinking about the 7th infinity stone.

What if avatar the last airbender had a 5th element such as gravity bending. The gravity bending could be balanced by a setback. For example to use gravity bending you have to sign a contract with a devilish spirit thing from the spirit world.

You could also make the MC the one giving the contract killing off the users to gain more power to eventually break free of its shackles or something.