Hey all, I haven't entirely read up on the Vortex story yet (still half like half of Minn's chapter to read through), so I don't know how much any of the proposed would interfere with what will still happen or might have been decided or whatever.
Anyway, rambling aside, I'll just propose the idea:
At some point in time, possibly after they destroyed the last vortex but nothing happened though they were expected to be all sent home, the group could tag along with Eris to her world -though obviously not intended (on her side). Perhaps a battle could've gone wrong as in, Eris created a portal to go back to her own world, thinking that she had knocked them all out or something, but was mistaken and they jump her, falling all through the portal en ending up in her world. Any other ideas are more than welcome, they just have to get to her world at some point ^^.
There, now that they are there, they find Eris' childhood home and the four graves under the tree, one of which is named Sakura. After some investigating or for whatever reason, they learn the story of how Avatar What'shisname (whose name they heard Eris say before -in the first chapter perhaps?) and how he lost his entire family -wife and three children- only to then be relentlessly hunted by this Eris person. However, since Eris had also called the Avatar her father, the group puts two and two together, and figure out that the grave marked Sakura is empty. How, don't know. Perhaps one of the earthbenders senses that there are no bones underneath it or something, but a doll. They could perhaps dig that doll up and take it with them.
Anyway, whatever exactly happens there, they figure out that Eris and Sakura are actually one and the same (and that the others are her mother and brothers) and that whatever happened, it has all to do with her and her father (d'uh). Since there are no more vortexes to destroy and they are still not home, they realize that the only way for them to get home is through Eris herself. Perhaps Ursa and/or Ling as the gentle spirits could propose to try to mend the bond between Eris and her father. They catch up with her at the moment that she is about to end her father's life. The team intervenes and tries to bring her to reason. Obviously, that wouldn't work as Eris' sole reason to live for years had been to make her father miserable and kill him for what he did to her. However, the group can somehow make her see that there could be more to the story than she believed there to be. They can convince her to go back to the day her father came home and found the four graves, and she then sees him break down over her grave as opposed to that of her mother's and hears him say all the things he wanted to say to her before about him feeling guilty and all that but never could.
She returns to the present (perhaps she took one of the team with her as a sort of hostage or something) and does not really know what to do know. She can't forgive him and still hates her father, as those years of resentment don't magically disappear just because he said he was sorry to her grave. However, killing him doesn't feel right anymore. It could then lead to some sort of open ending in that she does sent the teal home, but no one really knows what happened to her (or her father), or even knows if the threat of the vortexes is really over, thus possibly keeping the option open for a Vortex 2 if someone would be ever interested :-)
So, that is rather superficially explained where we might take the Vortex story. You know the drill: comments, thoughts, remarks, input, talk away in the comment section :-)