I think if it were the Gaang (not likely due to the Timeline) but just going with it; I think Zuko might lean towards the Shadowbenders. Understanding that things might be more complex like with the Fire Nation.

I don't think it'd quite be like the Great Divide episode (mostly because it wasn't quite handled right). I think Aang would have to find a way to bring both sides together. Then there is the understanding that the Avatar would have to find a way to incorporate both Light and Shadow into the Cycle of the Avatar. They'd need to find out why Wan never received either Light or Shadow Elements. As well as how Aang (or whichever Avatar would discover these two Nations) would receive these Elements. Would he/she get them from newly discovered Lion Turtles or some other way?

And then there's the fun idea of bringing both Nations to the surface. Those will get fun reactions!

I figured we could place this in between ATLA and LOK, given the large gap between them. Plus, we won’t have to invent a new Team Avatar this way.

Zuko’s a firebender, so I just figured he’d prefer light, at least in the beginning, over shadow (just naturally). Maybe he’d decide to help some lightbenders defeat a group of “enemy” shadowbenders, and get qualms over that. Surely the “good guys” should be more peaceful? It would remind him a bit of the earth kingdom soldiers from Zuko Alone.

As for the lion turtles, maybe they were just really stuffy and believed only certain people they deemed worthy could receive their kind of bending. Because of this, light and shadowbenders might be highly skilled in ways other than their bending.

Skilled in what ways?

The lion turtles might force anyone who wants to learn their kinds of bending to take tests that require physical prowess or intelligence to pass, so the first light and shadow benders would tend to be somewhat gifted. They’d pass down those talents through generations.

I think the training process would be extremely difficult, sort of like what you see in the Legend of Korra when Tenzin is training the new airbenders, only a lot more intense and the trainees would probably be a lot more excited.

Because darkness is just the lack of light, I think that in technical terms, shadowbenders would (technically) be lightbenders. The thing that would differ them from other Lightbenders would be how they use they power, earning them a different name. The shadowbenders would probably be very "everything isn't as it seems" "all the lightbenders can do is put on a show", and the main cause of conflict would be over moral between light and dark, and the avatar would probably face a lot of spiritual chaos over that, and the show would probably have to specify between "darkness" and a real "dark matter" type of thing. But if they would do so, would there be a large enemy who finds out how to bend 'dark matter'? Would there be a moral conflict over whether it's okay to bend 'dark matter'?

As for Team Avatar, I think timeline-wise it would serve well to be between ATLA and LOK, because the with the gang being more mature, they could probably face these things better. It would also help clear up all of the dark avatar stuff in S2 of LOK.

I also agree that Sokka would side with shadowbenders, or at least hear their perspectives. This would also give Sokka a lot of character growth and maturity, especially because the Gaang doesn't really take him that seriously most of the time, and probably think it's because of the superficial decision that he originally made to side with the shadowbenders (ex. he didn't like the food of the lightbenders, as someone already mentioned).

Saying a Shadowbender would be a Lightbender is like saying an Icebender would be a Firebender. Or an Earthbender would be an Airbender. That isn't how it works.

I think it'd make more sense after the Korra comics.

I think that there could have been a good explanation as to why Humanity didn't originally get either Light and Shadow by the time of Avatar Wan. I think due to the primitive nature of early Humans; there could have been a couple of incidents where the Lion Turtles would originally deny Humanity their respective Elements. Only until much, much later would Humanity have a chance to gain these Elements.

@SonoPazziQuelliRomani I don't think Shadowbenders could bend Dark Matter. Although, I'm not as familiar with Dark Matter and how it could connect to Darkness.

@Zealous Gamer I agree. I think the show has yet to determine what Dark Matter even is.

If these are going to be elements, they should probably have their own “original benders.” I’m thinking we could have some sort of two-faced, two-named spirit based around the Yin-yang. Both groups claim to have their own original bender, and they call the spirit by different names, but in actuality the spirits are one and the same.

Maybe the original benders are twins, who grew up close but slowly grew apart as their ideals became more and more 'extreme', leading towards the large separation between the two?