This is a topic me and my friends have discussed thousand of times. Its kind of interesting. The idea of bending/ an even higher level of bending (Theoretically speaking) could/should be the following.
Earth bending-
This is basically an ability over Gravity, the ability to use bend things with the frequency of Raw materials like Rocks and etc. Instory, its origin is from Bagermoles iirc. At the fullest extent of power, could an Avatar/Earthbender use their abilities to bend their bones to become lighter, or even attach themselves to earth to defy gravity (e.g somebody is able to walk up a wall with no effort at all.) (This might sound stupid, but its all just theory)
Water bending-
At full power, shouldn't an extremely powerfil Water bender be able to bend Air as well? Water is basically Hydrogen and Oxygen, Oxygen is the main gas found in Air. At full power, or even beyond full power, a powerful bender might be able to split the bonds. This might branch out further... What other Elements have Oxy and Hydro in it? A powerful bender might be able to create water through just this.
I also question why nobody ever further elaborated the potential Blood bending has. A blood bender could easily kill anyone by simply pulling out the water of the person. Remember when Hama revealed shes a blood bender, and pretty much destroyed a nearby tree by pulling out the water? That can also be done to a human, Amon was strong enough to do such an action. If it weren't a show targeted to Kids and Teens, this power would probably be used, or at least discussed much more.
Fire Bending-
Combustion man says hi.
His concept was pure genious, he figured out the true power of Fire Bending; Combustion. Though in my opinion, it was severally limited.
Can you imagine the level of destructive power that would have reached if he were around during Sozin's Comet? Dang
Air bending-
An Airbender might be able to summon winds that rip through your skin. Of course, Air benders might be able to bend Water as well.
These were probably already discussed/revealed to be false or w.e, but I just wanted to put this out there. Any thoughts?