<p>First, I don't think it's reasonably possible that these people could have had Avatars, at least not after the civilization was lost. Said Avatar would have to be discovered, trained, & live their entire life without the civilization's existence ever being discovered. That's really unlikely just from the normal training process, let alone the level of scrutiny the Avatar would be subject to later in life.
</p><p>That leaves either the possibility that the Avatar Cycle just skips over them because of their tiny population size & sheer random chance, or there is actually a mechanism behind it. I did consider omitting the Foggy Swamp Tribe from the latter theory & proclaiming them to be an exception, but it's also possible that, although they lived in the south pole, they have no relation to the modern southern tribe. They could have been an isolated ethnic group that left before the south became unified.
</p><p>But, more importantly I think, it's the logical consequence of Wan establishing the Avatar Cycle by learning from the Lion Turtles. If the Sun Warriors for example came from a totally different turtle, which seems likely given they have almost nothing in common with Wan's society, there's no reason for them to be connected to the cycle. This origin for the cycle is an implication many have picked up on, but I'd go as far as to say that it <i>must</i> be true.
</p><p>The 4 nations reincarnation cycle cannot be a universal, otherwise the nations would go through huge fluctuations in population, because their birth rates would necessarily have to reflect the death rates of the previous nation, & there are very significant population discrepancies between the 4. No, something had to happen that gave Wan a different reincarnation cycle than other people, & the most obvious solution to me is that everyone reincarnates within their culture. Wan, being effectively part of almost every extant nation-community, would therefore reincarnate cyclically into each of them. Which, as I mentioned earlier, in turn suggests that he cannot be reincarnated into the few communities that remain isolated from the mainstream 4 Nations.
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