Oh, there are plenty of things they've kept secret. Release date, trailers, NEWS. The last thing we need is more secrets.

A vacuum is empty of anything. No air, no matter, nothing. Nothing to bend. Airbending itself could create an air vaccum; an area empty of air, but that would be them removing the air from an area. By airbending it away.

Thunder is just noise. Sound. Possible. I like it.

A shockwave isn't really one kind of thing. Also hard for me to describe. It's like….a medium being distorted or reacting to a wave of energy. I'm pretty sure I've seen Toph create a shockwave via earthbending, and Aang create a shockwave through airbending. I'm finding it difficult to say what I mean about it….perhaps you mean to say they could bend kinetic energy? That could work. Would be overpowered, but I'm game.

Fartbending. No words….no words.

That's suction. As in pressure differences. Pressure differences usually caused by air. Air as in airbending. Denied.

That is…preeeeety much what they already do. And, sorry to burst you're bubble, but it isn't a perfect idea. For fire, lighening is the subskill. Fire and lightening, while alike in "HOLY MOTHER THIS IS HOT AND BRIGHT AND I CAN TASTE MY OWN MELTING FLESH!" could not be more different. Fire is combustion, an exothermic reaction. Lightening is the sudden balance of two electrically charged regions. Earthbending's subskill is metalbending. This one is downright funny. They bend dirt and raw minerals. Not…not actual metallic elements. They bend the impurities within the metal to move it; not the metal itself. Bloodbending is quite similar. The waterbender takes control of a quantity of water that they cannot see, held within a foreign object. Blood actually is basically water with extra goodies for life support. So they bend the water. In the blood. In the person. My point is that out of the three subskills, two are technically super-focused versions of the orginal art, tailored to use an element to control something else. The other is something…well, the show uses a more primitive definition of "element," so the science is understandably and acceptfully tossed out the window…the other is only similar in terms of a few characteristics. Fire's hot and burns things. Lightening's hot and can start fires that burn things. So two subskills are super-focus, the other is something remotely similar. So airbending's subskill would be superfocused into something else, or somthing remotely similar. Like I said though, telekinesis sounds off to me, because I bet a talented airbender could do that with such simplicity that it wouldn't be fit to be called a subskill.

A good subskill for air would be either what they're saying, sound, or possibly the classic Greek element aether, the essence the gods breathed, and made up heavenly bodies. I think the former would be better. Aether could be overpowered, or maybe a simple meditation tool, which would be in line with the Air Nomads.