This is the way I view bending techniques.
Each element has
1/ A high level technique
2/ A Bloodline (must be born with) ability that is rare
3/ A spiritual technique.
Fire – Lightning Generation/Redirection
Earth – Metalbending
Air – Flying
Water – Bloodbending
Fire – Combustion Bending
Earth – Lavabending
Air – Unknown
Water – Psychic Bloodbending/Bloodbending without full moon
Fire – Chi/Spiritual sensing technique used by the shaman in Beginnings Part 1
Earth – Toph’s seismic sense enhanced by the spirituality of the swamp allowing her to “see” Su and Lin even crazy far away
Air – Astral Projection
Water – Spiritbending/Healing
This is the way I group the techniques the only ones in doubt being Air’s rare technique that only a few people are ever born with and also Earth’s spiritual technique. I think we are beyond simply saying each element has a sub element, just because there are so many unique techniques for each element.
I think Air’s bloodline technique is most likely sound based, we have already seen certain airbending techniques involve enhancing sound like Aang using a bison whistle and so on. Sonic booms seem like a powerful combustion bending type technique.