The Avatar franchise’s titular ... gained control over the Avatar state, something that would only come with their mastery of all four elemental bending arts. Traditionally, the Avatar would ...
Avatar: The Legend of Korra might have cued fans in about a key curse that follows they Avatar through it's reincarnations.
Some Waterbenders in Avatar pushed Aang and Korra to their limits, using deadly skills like bloodbending to challenge the Avatar State.
Picturing that climax with both a spiritual version of Korra and Aang appearing within a new version of the Avatar State, and that could make for a heck of an Avatar: Seven Havens episode.