Unfortunately, as the Air Nomads' culture and bending was never covered in depth, this would be purely speculation.

The idea that a highly-skilled Airbender could control a person's breath is an interesting one, but is counter-intuitive to the pacifist nature of the Nomads. There has been speculation that Airbenders could create a vacuum, but as bending disciplines seem to follow the same metaphysical laws that their counterpart substances here on earth, a vacuum would only be able to be created in an airtight environment; otherwise, more air would just rush in to fill the gap and the bender would only succeed in creating a temporary source of low pressure.

Along those lines, and with what I know about meteorology (still assuming that the weather works the same there as it does here), a skilled Airbender could have a significant effect on the weather by creating surfaces of low and high pressure at different altitudes; however, as I haven't seen a meteorologist in the A:tLA universe, this seems unlikely. (I will note, however, that Katara and Aang both showed a hint of meteorological knowledge when they surmised that both of them could shape clouds because they were "water and air")

If one is to speculate as to what the "ultimate form of Airbending" is, one must break down what Airbending actually is; aerokinesis, or the ability to manipulate air with one's mind or, in the case of A:tLA, by channeling and manipulating the "chi" of the air in some way. Now, to consider what could be done as far as "ultimate Airbending," one must take into account that, while we know a lot about how air and wind work, the people in the Avatar universe might not; so, it is entirely possible that the "ultimate form of Airbending" could have something to do with highly compressed air.

Keep in mind that Aang is a unique case because he's the Avatar, so unless another Airbender springs into existence (which won't happen), we'll be stuck in the perpetual cycle of speculation.

I agree with kai on many of the points.

However, I don't believe there to be an ultimate air bending form. Airbending in and of itself is complicated and more powerful than the other bending forms. How they could have lost if they could compress air, create vaccumes, et cetra, is completly beyond me, even if Sozen's Comet was to come. Fire needs air, creating a vaccume would dissipate the flames before the firebenders could harm them. Creating a high pressured barrier between themselves and their opponent would also put out the flames due to too much oxygen surrounding the fire. Remember that the firebenders wiped them out, so it only stands to reason airbenders wouldn't have been able to stand up to them with the comet's advanced power and would do anything to survive... even kill. Controlling the enemie's breath might be plausable... but then again, it would also be too powerful if the airbenders could do it. Shoving air down a person's throat and killing them that way is probably much more effective than stealing the air from them and letting them suffocate.

I believe that this discussion is a neverending speculation as kai said because the Avatar spends his time mastering all four elements rather than go into one in so much depth they reach the advanced form of the bending arts that no one would be able to study the airbending in so much depth that they reach that 'next level'... even if they were born to a certain nation.

However, what I'm wondering is what the next Avatar after Aang would do to learn airbending! Since there aren't any airbenders left, he would have to rely on himself to rediscover the ancient art as well as the avatars before him. That might actually be a good plot for when we finish the Canon Plot! ^^ (just an idea XD)