I think the avatar cycle is destined to not go forward, after all the network will never allow spin offs or series so Nick will probably force the creators hands and force them to end the series with Aangs death and no chance of expanded universes or spinoffs.
The series isn't ending at the third book because of Nick putting the kibosh on it: the creators planned on a three season storyline from the very beginning. Also, they've said they're working on a new project within the avatar universe (aside from the movies) which suggests a spin-off is in the works; I doubt they'd be working on that if didn't have at least tentative approval from Nick.