The Gaang in the world of Berserk.
The Gaang (Aang, Katara, Sokka, Toph, Zuko and lets give them Iroh and Suki as well) as they are at the end of the series are transported into the world of Berserk to replace Guts before he meets Griffith. Also everyone else in Guts' party is gone, as the Gaang have taken their place in the story (but they all still start out together). How do they survive in the new world or do they even survive at all? What is the comparative power level of the Gaang and the world of Berserk? How much will Griffith be able to manipulate them? Will Aang succum to murder in order to survive and protect his friends? Etc.
Bonus round: Azula replaces Griffith (but still seduces Charlotte 😉 ).