To be fair, not a whole lot of options exist for an Air "upgrade".>Fire: lightning/electricity>Water: ice, and controlling plants and bloodbending if you count them>Earth: metal, lava
And what's Air supposed to be able to bend, aside from air? Not a lot would fit it. But Air as an element is constantly described as being high up and detached from the world, so a cosmic connection of some sort makes sense.
I know the show's all about balance, but Air was always treated as the "special" one. Right down to the color theme, the other three Elements are red, green, and blue, whereas Air is yellow. Air is like the detached, unbiased leader of the rest of the elements.