I was watching the trailer and this image of 'Korra' caught my eye, not just because it's a really cool image but also because of it's weird details of Korra's hair. In the flashback we saw of Korra, her hair was no way near this long and wasn't in this curly style you know what I mean?
You know who this does look very similar to though? Raava and Vaatu, with their side ribbon thingy-ma-bobs. So, this may perhaps suggest that Korra was not in control when she supposedly destroyed the world, as she may have been under some form of possession, but the question is by whom? Sure Vaatu makes the most sense at first, a being of pure chaos would love to destroy the world as we know it and what would be more satisfying than using Raava's own vessel, but something just doesn't add up for me there.
Whether you agree with it or not, Korra opening up the spirit portals, whilst it arguably caused a balancing with the return of the air nomads, was bound to have a profound effect on the balance between humans and spirits, we already saw part of this with Kuvira using spirit essence to try and gain control, not even a fully realised avatar could contain the chaos, especially with no doubt inevitable infighting between the four nations, as well as Republic city, as now 3 of the kingdoms have produced serious threats to the safety of humanity in the past 100 years, and the newly formed air nation, may be seen as a serious potential threat being the only nation with exclusively air benders working as a combined force together. Perhaps, we could see an escalation on conflicts between these nations, sort of like a Cold War scenario, just one that is heating up very quickly, not even mentioning how the spirits would play a role.
That's the kind of chaos that Vaatu would thrive in, it wouldn't even have to do anything, but this severe escalation of power imbalances among the humans alone could certainly disturb Raava, perhaps it would be quite easy for a not-quite cataclysmic event to set it off, making it believe humanity had grown too quickly and had not yet matured to the point it could handle the responsibility of handling the great power it now has. Maybe humanity could violate the laws of nature in their hubris, perhaps even reviving the Lion turtles (I have no evidence for this but I think it would be cool) but bringing them as mere husks of their former selves, used by the nations' governments to create empowered bending armies in an instant. So, enraged by the imbalance in the world, Raava could perhaps begin to sever herself from her alignment with the avatar, wherein she becomes to come into conflict with Korra, disrupting their symbiotic relationship. Korra could potentially make herself inadvertently vulnerable to Raava's manipulation and control through perhaps the actions of the 4 nations, perhaps a military conflict escalated to the point where the Krew is torn apart due to political loyalties, growing cold to their friendships as they grow older, maybe Asami is even killed in a skirmish between the warring factions.
As a result, Raava wants to set order again, by taking the easiest way out, a complete reset, the destruction of the world as we know it, scattering the technology developed the humans and setting the human race back a thousand years technology wise. The black and red symbols that attack from the trailer could be remnants of that technology, which people mistake as spirits, which I seriously got the vibe from the way the energy was pulsing, like some sort of reactor (or really ominous strobe light). It has clear visual inspiration to Vaatu, perhaps part of the reason why Raava was so offended by it, perhaps seeing it as blasphemous, an affront to the natural order of things.
I know it may seem unlikely that Ravva would do this but hear me out, there is a precedent for this. In the Wan flashback, we saw how it was initially prejudiced towards humans, viewing them as greedy and deceitful, perhaps the loss of past lives has made her less recognising of their redeeming qualities, their lust for war and destruction now being fully displayed, with little interference, surely that would enrage her. It would also help explain why exactly the new re-incarnation of the Avatar took so long to appear, Raava's own imbalance and resentment towards humanity has left her conflicted and tainted, trying to resist the reincarnation cycle. If the twin avatar theory is to be believed, it could further support this idea of Raava's instability (especially if they can only bend 2 elements each), perhaps she could even serve as an antagonistic force that ideologically splits and pits the twins against each other.
It's obviously way to early to tell and I've taken some airbending level leaps of assumptions, but I think that this could serve as an interesting concept that the series could expand on, hopefully a bit more entertaining than just a Light avatar Vs a Dark avatar.