Atla_Sanity is mirrored here on DW, but I can't remember if you're on that comm over on LJ. It is quiet nowadays, but I know it will pick up again once more stuff happens with the fandom.

Capslock_Atla has also recently been mirrored.

Avatar is the simple-named second comm over here.

Donebykorra is a neat little rewatch comm that started when the final airing date became known. I haven't visited lately so I'm not sure if they ever finished the series, but it's nice to read and catch up on if you ever want to.

That's all I can think of at the moment. Sorry!