Aang, master of all 4 elements suddenly finds himself in Ultima I called by Lord British to defeat the Immortal Mondain. Assuming he survives, he gets called back for all the other Ultimas.
Meanwhile Spoony... I mean the Avatar from Ultima shows up in the Avatar the last Airbender world, and is instructed to stop the Firelord and bring Balance back to the Universe.
How do they do?
Round 2: Korra, and the Legend of Korra-era Avatar world instead.