Legend of Korra open up some ways to continue on avatar seires. As the spirit world and human world marge togather, some evil doer/s find a way to manipulate spirits(as we saw it in book 4). The next avatar (me also prefer Earthbender :3) fight against them and shut down the gates of spirit world again. In this time period the necessity of any bender will be no more needed, as the technology surpress that power. So the avatar will lock him/her self in the spirit world,which can lead to a story of last avatar.
Or
The story of new avatar legend can begin with clash between avatar and dark avatar. Book 2 make a possibiltiy to bring down dark avatar character. As many years pass, dark spirit - vaatu resurrected again, he soon took over the spirit world.Then chose an avatar to destroy the human world. This time a firebender will be seen as avatar to unleash the rage which have been created for destroying the spirit world and almost destroying the human world. Afterall there's nothing can be stronger then the fire of rage.
Or
They might make them both
It's all fiction. Whatever the storyline they make, i will love to and want to see a new AVATAR. Many people may think any more series will ruin it's reputation, me also think about the same when they create Legend of Korra. But after watching it, i had to say - that is awesome!!! So , i believe if the creators think about it, who knows we might go crazy again.