Fire Nation:?? Agni and Surya, the fire and sun gods (respectively) of the Hindu religion.
Water Tribe: Poseidon (water god of the ancient Greeks) and Khons (moon god of the ancient Egyptians).
Air Nomads:?? Indra and Rudra, the sky and storm gods (respectively) of the Hindu religion.
Earth Kingdom:?? Still up for grabs, since most ancient religions didn't have any "earth" gods.?? Any suggestions?