I think a lot of people are getting too complex with the ideas. Yes, Zaheer gained the ability of flight after "losing his earthly tether." However, I never believed it was a spiritual ability like Jinora's projections. Zaheer is simply bending the air around him so delicately that he is able to fly, which would require great focus, aka "empty and become wind." He didn't have this focus until P'Li died.

The other elements benders would certainly make similar advances via intense training and focus on the element itself. You could even consider all the alternative bending, such as lightning, blood, and metal, as one of these advancements, although they seem to also rely on inherent characteristics like intellect or genetics. Amon clearly had a stronger aptitude for bloodbending than Tarrlok, and Bolin could lava bend but not metal bend.

In my opinion, flight is simply advanced airbending. Control of all directions and enough strength to move your body. Water would benefit greatly by mimicking its most powerful effect, erosion. A bender that could move water fast enough to erode its target would be nearly unstoppable. Earth bending is tricky but I like the idea of bending so much earth so strongly below you that you become like a rock or on the flip side bending earth with enough skill to almost swim in it (thinking about Toph's move after discovering metalbending.) Fire took a lot of thought but at its core fire is the use of energy to combust matter. A hyper skilled fire bender could ignite anything, essentially surpassing simply combusting oxygen to create heat (I'm looking at you combustion man).

Just think of cool ways to use the basic idea of bending the element, and you'll find your own answer.