Here's one: Avatar 3000
In the year 3000, Avatars from worlds throughout the galaxy travel to Avatarworld to train to be the Avatar of their respective planets.
So there's an airbender from a skyworld, a waterbender from a waterworld, a firebender from a volcanic world, etc.
A talking sky bison teaches air bending, a talking dragon teaches firebending, etc.
The students have to fight evil across the galaxy.
The master trainer is a descendant of Momo, and he talks, too.
The headmaster is the lion-turtle.
Nick wouldn't be interested, but I bet Cartoon Network would. (And then they'd mess it up!)