I've mentioned this several times before, but they already explained in one of the panels that the ruling council is comprised of an Airbender (Tenzin), a Waterbender, a Firebender, and an Earthbender. It's the central conflict in the series, I don't see how this could have been an afterthought.
No, I understand that. I'm talking about the first series, Avatar TLA. Did they already have this idea then about this conflict arising or came up with it after that series ended? I'm not saying that there is no room for tensions between benders and non-benders, there clearly is. I'm just noting that the division between bender and non-bender is sort of complicated because they are already so mixed together.