I’ve seen a couple of Voltron Avatar AUs, and overwhelmingly the consensus is firebender Keith (which makes sense, considering his element, and the galra) but if I’m going to split hairs here: a significant part of Keith’s character is there’s a lot of his history and himself that he doesn’t know, and I take a little issue with the idea of Keith being overtly raised in the fire nation.
Now that’s not to say you couldn’t swing it, but consider:
Keith who grew up in Earth Nation and has always just figured he’s a nonbender because he’s clearly no earthbender. And per canon he’s an orphan so it isn’t as if he can reference with anyone.
Keith who’s kind of the team Sokka except instead of the boomerang guy in the middle of all of these elemental wielders he’s the sword guy for a long time. (and I like the idea of nonbender Coran as well so, hey, solidarity is exciting) and then the first time he firebends nobody is more confused than Keith.
Bonus points: Keith even after the revelation forgetting he is a firebender. Several times. “What do you mean we can’t start a campfire?” “I don’t know if you’ve noticed, but it’s pouring rain outside. All the firewood I could find is wet, we have to wait for it to dry.” “can’t you just-” “…oh. Yeah. right.”