Interesting idea, but I don't see it. The Guru is there as a spiritual guide for Aang. He will not be a skill guide, but instead I think he will most likely help Aang find reveal a secret about himself. I believe it is going to be an inner exploration. Pherhaps Aang will find something inside himself. Or maybe he will help Aang find out about what happened to the other Airbenders... Anyway, all I'm saying is that I personally don't think he's going to be a bender. He is simply going to be a guide.
P.S. This is kinda off-topic, but who says you have to be a bender to be powerful? Remember the YuuYan Archers? They were TIGHT!