Avatar in conversations here refer probably to Avatar: The Last Airbender, the Nickelodeon cartoon, which probably would have a bigger appeal than South Park : fantasy setting heavily inspired by Asian mythology and legends, with a touch of steampunk, cool-looking abilities and powers, a broader audience (as the chant says, from 7 to 77), a huge success, etc.
Although I wonder how they would have made the choice of gender or 'bending' work if we played as the Avatar, seeing that if we stayed with the show's 'canon', there would be at least one generation between each choice.
You may be overestemating Avatar:TLA here. Strongly. Its a big IP, true. Nowhere as big or well known as South Park, though.