Avatar could easily contest to being the greatest TV show of all time. Thats not hyperbole: you can't watch The Sopranos with your kids. It dealt with deep themes of culture, racism, war psychological trauma and environmentalism in the ways the best of Miyazaki does and doesn't shy away from showing the results of genocide in a way kids can understand. Yet at its heart is still the humour and action you would expect from an adventure series.

Korra went further down these tracks with more adult themes of religion vs atheism, anarchists and the totalitarian response to terrorism. And it introduced Lava bending which was just awesome!

If you're after more modern series with similar sensibilities I would thoroughly checking out Netflix's Voltron reboot which is magnificent.