On February 21, 2005, Nickelodeon aired the first episode of Avatar: The Last Airbender, an original animated series about a kid named Aang with the power to control all four elements — earth, air, fire and water — and who had to use that power to bring an end to a brutal war. The series ended in triumph three years later, with sequels and remakes coming out in the years to follow. Now, almost 20 years to the day after the original show first aired, a new sequel show has been announced: Avatar: Seven Havens.

The original Last Airbender show followed Aang as he tried to make peace between the Fire, Water, Earth and Air nations. The sequel series of The Legend of Korra picked up with the Avatar who came after Aang; in Korra's time, the people of the four nations were all mixing in Republic City. Seven Havens will pick up a young...