Seven Havens Theory
So I have been thinking a lot about the new show, and all I just finished SVTFOE, and its ending gave me an idea.
What if Sozin's comet, or something like that, becomes a threat to the nations and coincidentally strikes Republic City? In the ending of LOK, we see how the cannon becomes amplified because of the spirit vines so what if the vines make the comet more powerful or make it attracted to it making the comet strike the portal or cause another explosion.
An explosion this strong would devastate the entire planet and realm so what if Korra had to
A: Merge the two worlds to give both a chance to survive
Or
B: The explosion hits which causes the 4 nations to become the 7 havens, the possible bender storage, and also damage to the spirits' world
This is more than likely HIGHLY unlikely but it got my dusty ol gears turning.