It has been revealed that all 4 Air Temples will be visited in book 2, but why? As stated by Tenzin in "Rebel Spirit" he and Korra had planned the trip in an effort to connect with past Avatars and strengthen her spiritual side. Well, now that plan's mut, Korra is training with her uncle Unalaq (in the NTW?). So now, Tenzin, Bumi, and Kya are apparently going on this adventure. What could they possibly gain from this experience? Is it an attempt to reconnect with their father's spirit? Is it a sibling bonding trip? And what goes wrong? You've seen the screen shot of their bewildered faces. You've seen them fall down a waterfall in the trailer. Who or what did they discover, and is it threatening them?
To be honest, my creativity is drained. Artist's block has left me a dried out husk of the teen I used to be. But, as bad as what you are about to read may seem, if you simply add on some decent ideas, it would make sense. So, Korra canceled her involvement Tenzin's trip. He's upset, so his siblings pester him into revealing what is wrong. When Bumi learns there is an "availiable seat" on the trip, he wishes to attend. Naturally, seeing that this bothers Tenzin greatly, Kya also wishes to go. Tenzin denies this, so of course, they come anyway. They depart from the North Pole, and a certain spirit takes interest in them. This spirit decides to follow them on their journey. The product of Kataang find out much about the Avatar on their quest (possibly leading into the "Begginings 1") but they also learn the intent of the so called "Dark Spirit" and have to go warn the others immediately (others meaning Team Avatar). But this "certain spirit turns out to be the Dark spirit's assistant or something and must now prevent the product of Kataang from delivering the news that could potentially destroy its master.
So yeah, that's my theory, feel free to add your own in the comments below!