A small, mulitplayer game like one that’s just pro-bending would be neat.
I’d play it, and I bet a lot of other people would as well.
However, it would require actual thought and creativity to pull off successfully. It’s not the kind of thing that the market analysts would greenlight because there’s no data point showing that it works.
Meanwhile, a generic fighting game that just has Avatar characters is an easy sell. No risk. No creativity. Just cold, hard, data.