Naturally, the Avatarverse is mostly Asian, with some Pre-Columbian influences as well, and since LoK's premiere, we also see certain Western (modern European + US) influences, ranging from clothing to technology. However, that's not what I will attend to.
What it is I will attend to is something the potential existence of which I realised as I read about Norse mythology just after Book II's end: a connection between the Spirit World - as it was revealed to be in LoK - and the aforementioned Norse mythology. Before you start discarding the idea as rubbish, please let me explain:
First of all, potential parallels in the area of cosmology:
The Earth could be compared to Midgard, the world of humans in Norse mythology, which lays in creation's centre. Basically saying, as a lack for other "worlds", the Spirit World would unite the others, namely Asgard, home to the Æsir (a "family" of gods), Vanaheimr, housing the Vanir (another "family" of gods), Jötunheimr, home of the Jötnar (giants; enemies of the Æsir, and probably of the Vanir too), Muspelheimr, the world of fire, Niflheimr, the world of ice, Svartálfaheimr, home to Dvergar (dwarves), Álfheimr, which houses the Álfar (elves), and maybe even Hel(heim), the realm of the dead. Naturally, that's a very vague comparison. However, there is a certain parallel formed by the two polar portals and the Norse Bifröst, the rainbow bridge between Asgard and Midgard, as well as the tree Yggdrasil, which interconnects the Nine Worlds, however that one would be fit more as the Tree of Time, as it is, well, a tree.
Second, the battles between Korra and the Dark Avatar and between Korra's allies and the Dark Spirits and their potential parallels with Ragnarök:
Well, to start with, the whole conflict mirrors Ragnarök: While in the events of Ragnarök, the Gods as forces of good have to defend themselves against creatures of evil, mostly giants, the wolf Fenrir and the serpent Jörmungandr, in the Battles during Harmonic Convergence, Korra and her allies, as a representation of light, therefore "good", clash with the dark, thus "evil", creatures of the Dark Avatar and his dark spirits, which forms a clear contrast to what he had known to be the Avatar's duty till then - to uphold and restore balance. However, that's not the similarities' end - by far not. In both cases, the fate of the respecitve worlds is decided: in the Ragnarök, the gods fall and thus, give way for Midgard - the world of mankind - to be flooded, even though it later emerges again, whereas in the second book's closure, the fate of both worlds is decided, as either of them threaten to fall into an era of darkness, which is basically the same fate that Midgard awaited after Ragnarök.
Now, as for the combatants: Korra could be seen as a female version of a combination of Odin, the supreme god, Thor, the god of weather and alike, and a son of Odin's named Víðarr. The Dark Avatar/Unalaq would thus combine the roles of Jörmungandr and Fenrir, the former is also called Midgard Serpent, which explains the nature of its being quite well, the latter being a wolf of pure evil. The reasons for those: Odin simply because he is the highest of the gods, and as the Avatar is a union between a human and one of the two highest spirits, it seems quite logical. Furthermore, in the events of the Ragnarök, he is swallowed by Fenrir, like the Dark Avatar kind of annihilated Raava for some time. As for Thor, it's because of slaying and being slain by the Midgard Serpent, Jörmungandr. Now for the likely least known, Víðarr, well, he slays Fenrir, quite like Korra slayed the Dark Avatar. As for everyone else, all allies of Korra's would basically portray the Æsir and their allies, whereas the Dark Spirits would be an Avatarverse version of the giants and other enemies of the Æsir.
Now, with those tons and tons of text, what do you people think about what I proposed here? Share your opinion on my theory, and also those of yours about the source of those rather unusual theme!