Word "Avatar" it is most ancient word from sanskrit language, which is base of both buddhism and hindusim. Word "Avatar" mean incarnation of holy being or in this case it is for "Aang".

Lord Buddha was 9th Avatar of Earth spirit. There is 8 other avatar too but they have there own story and some are famous in asia. All his story is written in ancient book name"Vedas". And even the Buddhist monk got major subject in there study which is "Vedas". Every animation creator got there ideas and motivation from something and in my view "Avatar The last Airbender" must come from same view and angle.