I'm not talking about just studying really hard in hopes of learning bending. I'm talking about something more fundamental than that. I'm talking about a serious level of discapline where one would see the mechanism behind bending, truly understand it and then learn to apply it.
How does this relate to my suggestion for Korra's journey? Korra can bend all four elements, something no one else can do. Then she lost her ability, then got it back. How did this happen? I don't know and she doesn't know either but her ability came back and all is good. I thought a good journey for her might have been to go through what I described above… First she (possibly with the help of a guide) reaches into herself, perhaps through the discapline of meditiation and finds the exact mechanism that allows bending to work, then having seen this mechanism, she truly develops an understanding of what it is and how it works and is able to recreate it. This way, she doesn't just get her bending back, she knows it and how it works so that even if it were taken again, it wouldn't be gone because it's too much a part of her to be lost. In this, she grows as a person and as a powerful avatar.
How does this relate to anyone learning to bend? There's a mechanism involved in bending. If Korra can reach into the secrets of life, then others can too. They can see the mechanism behind bending, understand it and learn to apply that understanding. If they have the same innate ability that any bender or avatar has, that makes things easier. If they don't, maybe they can study this missing ingredient we call "innate ability" and recreate that too. In this, bending becomes a real science that can be repeated instead of a spiritual thing that only special people can do.
That's a good analogy. Let's grow some legs then.
Just by studying/practicing? How the heck is that supposed to work? You're willfully missing the point of the analogy. Study does not create the potential, it just develops what was there to begin with. If the potential isn't there, it can't be wished into existence.
Anyone can learn kung fu. They have it in them. If they don't have legs, they'd need to go a step further and get some. They'd need to learn something about genetics, cloning and a lot of other medical stuff in order to get those legs but they can learn kung fu. Bottom line is that it's possible for anyone if they understand the mechanism and have or develop the means. That's what I'm getting at and I imagine that developing bending is easier than growing legs. In fact, it probably could be "wished" into being because as a spiritual force, the work would be done on the inside.
Except that you're choosing to ignore the parts of it we do understand, the rules that have been very clearly and consistently spelled out over four seasons. That's where your argument falls apart. The fact that there are gaps in our knowledge does not mean that we can blithely disregard the knowledge we already have. Just because we don't know how to unify gravity with quantum mechanics, that doesn't mean you can declare gravity imaginary and try to walk off a cliff.
I wouldn't declare gravity imaginary and walk off a cliff. I'd try to understand how gravity and quantum mechanics work so that I could understand and develop a mechanism involving antigravity before stepping off.
Let's consider this. At the end of A:TLA, Aang was the only airbender left in the world, and he'd discovered how to energybend, i.e. to take away a person's bending. It logically follows that he would have tried to use energybending in reverse, to create airbending ability in people who didn't have it. Given that the entire Air Nomad civilization was in danger of ceasing to exist and throwing the world permanently out of balance, it was absolutely urgent that he do everything he could to create new airbenders. And he had decades in which to study the problem. So if there were a way to create new airbenders other than standard procreation, Aang would've had a very, very powerful incentive to discover it. And the spiritual forces of the universe would've had a very good reason to make sure he did discover it. But no such method was ever found, despite the urgency of the population crisis. Which makes it quite unlikely that any such method exists. If it could be found, Aang would've found it.
Lots of scientists and spiritual seekers spend entire lifetimes searching for answers and never find them. Doesn't mean that they aren't there. Maybe a means does exist to create other air benders it's just that Aang couldn't find it in his lifetime. The fact that someone can bend air suggests that a mechanism exist to make it possible, it's just that no one has found a way to duplicate it.
We saw some bending in Book 1 that wasn't possible before.
But nothing that violated or erased pre-existing rules -- merely new discoveries that added to them. Metalbending is just a refinement of earthbending; it works by sensing and manipulating non-metallic impurities within metal, so it's really just earthbending with very small quantities of earth. We saw last season that a highly purified metal is immune to bending. Bloodbending, similarly, is just waterbending with the water in blood. They're new applications of existing principles -- they aren't violations of those principles.
I was thinking about when Amon was able to bloodbend without a full moon. He bent the rules.
Besides, if anybody could become a bender just by practicing really hard, you could potentially have multiple Avatars running around.
Not just "study hard" as I said. But yeah, everyone would have avatar-like abilities. Or at least everyone who wanted them.
If a non-bender can teach themselves how to bend water, what's to stop them from teaching themselves how to bend the rest of the elements too?
Nothing. You'd have a total paradigm shift. Bending and avatar-ing would no longer be what the world thought it was.