And it's spoileriffic, so here's a paragraph of text. Paragraph of text. Paragraph of text. This is a paragraph of text. Is this enough text yet? I can never tell if this is enough text. I've got a fever, and the only prescription is more spoilers.
Reddit hates this theory apparently, so let me share it with you guys to see if you think I'm crazy.
So as of Beginnings, we now know not only is the Avatar the Avatar of Raava, the spirit of light, but also that there is a dark counterpart to Raava, Vaatu. They're so opposite, that they're literally just the same design with inverted color pallets.
Vaatu was sealed away ten thousand years ago by Wan, the very first Avatar, and his next shot was ten thousand years later, AKA now, fighting Avatar Korra. So Vaatu hatches a scheme to win this time, using the same power which defeated him last time, the power of the Avatar, or in this case, the Anti-Avatar.
So when Avatar Korra is born (the Avatar Vaatu knows he'll most likely have to fight), Vaatu hatches his plan. First he contacts Unalaq. How he does this, I'm not sure. Still, it wouldn't be too far of a stretch to assume that merely being nearby Vaatu, even while sealed, is enough to corrupt a spirit to darkness (in fact, since Vaatu's tree looks so much like Koh's tree, who is to say that Koh isn't one of the dark spirits corrupted by Vaatu?), and manipulate it or command it. Or perhaps during one of Unalaq's trips into the spirit world (we saw that he can do this in The Sting), he found Vaatu, or even freed him.
Unalaq has already shown that he values power over all, having betrayed his brother (Korra's father) years earlier, so Vaatu promises him power beyond his dreams. Powers to match that of the Avatar. Unalaq agrees.
Years later, Unalaq comes to Korra, and tells her that the portals at the north and south pole need sealing. Now, this part is a little bit trickier. When Korra returns home with Unalaq, he reveals that he's invaded, and is in fact a pretty bad dude! This is part of the plan too. All of Unalaq's actions, and the reason Unalaq is such an obvious villain, is because Vaatu wants Unalaq to be an obvious villain. He wants Korra to hate Unalaq, to the point where she'll do the opposite of what he wants her to do (seal the portals), to thwart his plans.
Vaatu somehow contacts Jinora (he's an eldritch abomination, I'll say he's telepathic), through Wan's statue in the premiere, pretending to be Raava, or some good spirit on her side from the spirit world. He tells Jinora that Korra needs to journey to the spirit world to fight Vaatu. However, Vaatu's real plan is to make Jinora the Anti-Avatar.
Jinora seems like the exact kind of person Vaatu would want to make an Anti-Avatar. She's pretty much the opposite of the current Avatar. Korra is headstrong and impulsive, Jinora is levelheaded, and kindhearted. Korra values action and power, Jinora values knowledge, and planning. The element Korra took the longest to learn, Air, came very naturally to Jinora and her siblings.
Not to say any of those qualities are bad, in fact those are a lot of the same qualities Aang had, but if Korra is the avatar who will be around during the Harmonic Convergence (which basically means she's the most important Avatar since Wan, at least to Vaatu), it makes sense that Vaatu would want to make an Anti-Avatar who countered Korra's skills. Not to mention, Jinora is someone who Korra cares about personally, and would hesitate to even use her full force on.
Korra gets amnesia for an episode, Beginnings happens, and she head to find this "guide," who is probably Jinora.
Jinora is smart, and not evil, so for Vaatu to possess her, it would have to be through trickery and force. The trick, however, is how to get Jinora near enough to possess. This is what his endgame is. He's tricked Jinora and Korra together, through the statue for Jinora, and Unalaq's treachery for Korra, into both deciding to go into the spirit world, and reseal Vaatu. When the two of them journey into the spirit world together, Vaatu manages to posses Jinora by force, and turn her into the Anti-Avatar.
Korra has to confront the possessed Jinora, who she'll have to kill/imprison in order to prevent the harmonic convergence from ending with Vaatu's victory, and that's the climax of the season.
Naturally there are a couple problems with this theory, but I don't think there's anything too large for them to write around, and I think it would explain some things. It's probably wrong, yes, but that's true of every fan theory ever.
So please, tell me why I'm wrong.