>>40044 (me)
Ok here's my sort of write-up/brainstorm of a Korra s1/2 with the bending serum premise.
For starters, I would go all in with bending ability and genetics being tied to nature and spirituality. With the industrial revolution, bending became rarer, and those born with it became weaker than those born in the past as mankind's relationship with nature became more and more extractive. This being a problem society is well aware of, Hiroshi Sato, still being an equalist in secret, is working with them to create a method of "extracting" and redistributing bending among the populace with a set of devices of Asami's design (driving home the Luthor parallel of her both being super-smart and having delusions of grandeur, as well as the communist influences of the equalists wanting equity for all), rather than simply super-chi blocking it out of existence (that technique never made sense to me tbh). The initial trials fail, as while they do successfully find a means to extract it, the raw bending energy would cause horrible disfigurement/death in those that use it, as well as kill the bender (usually bending gangsters kidnapped for experimentation) it was extracted from. Amon and his followers steal the tech and defect anyway, modifying it and using it much like in canon to harmlessly take away bending, with plans to destroy bending altogether, as they truly believe the world would be better off without it. When Asami befriends Mako after learning of the Avatar being the Fire Ferret's replacement team member, this is eventually revealed much later on to be part of her larger plan to extract the Avatar's bending energy and study/test it to see the difference between raw bending energy and the Avatar's obviously unique bending energy. When Amon eventually accomplishes this, Asami has the serum stolen and tested in secret, finding that it in fact works. This will ultimately lead up to the reveal of the Avatar spirit being a fusion of one of the most powerful spirits and a human long ago. The spirit plot of season 2 would begin with Future Industries capturing and experimenting on spirits in the material plane, as Asami understands that the Avatar possesses spiritual energy not found in typical benders, leading to a plethora of spirit attacks on humans. Korra, traveling with Team Avatar and Tenzin to re-learn all the elements from the various natural teachers (i totally stole this idea haha), also begins encountering these spirit attacks. Upon her arrival to the South Pole, Korra would eventually learn from Unalaq that the spirits are angry at humanity for their actions in perverting the balance of the world. He eventually tells Korra the true origin of the Avatar instead of it being a flahsback out of nowhere, himself learning it from Wan Shi Tong's library. Unalaq's ultimate goal in S2 will be to extract the Yin spirit Raava from Korra and reunite it with the Yang spirit Vaatu, rather than fusing with Vaatu himself. In Unalaq's view, the world was permanently thrown out of balance when these two separated, with him colonizing the South partly being to accomplish this goal by controlling both spirit portals, as well as his internal prejudice against them for neglecting the spiritual world(idk much about this plot I've only watched s2 like twice, feel free to add to this/critique it). The reason for this separation, via records kept within Wan Shi Tong's Library, being that Vaatu viewed the lion turtles' actions to be a great betrayal. In Vaatu's mind, humanity could not be trusted with bending as he viewed them as lesser than spirit-kind in every way, via the actions of some of the first Fire and Earthbenders waging war with spirits and eachother, while Raava favored humanity for their ingenuity and harmony, as displayed with the first Air and Waterbenders working with and thriving amongst the spirits and nature. This schism would ultimately lead to the separation of these two great spirits and plunge the world into the first great war, with Vaatu rallying an army of spirits to his cause and Raava, being powerful enough, gathers up the 4 elements and fuses herself with a progidy firebender (Wan) to galvanize the population of human benders to fight back. Humanity ultimately wins out, and the Avatar, with help of the excess of spiritual energy via Harmonic Convergence, seals the spirit portals, effectively banishing Vaatu and the majority of spirits to the spirit half of the world, forever separating the two, with the Avatar, in a compromise, vowing to be the bridge between these two worlds. Vaatu accepting this, would vow to return during the next Harmonic Convergence and eradicate humanity if the Avatar failed in their role. After Unalaq sacrifices himself to pacify Vaatu, Korra would ultimately convince Vaatu that the only way to accomplish harmony would be to reunite Raava and Vaatu within herself and end the separation of the two worlds, to which Vaatu agrees, revealing him to having missed Raava and learning from various spirits 10,000 years worth of deeds and actions Vaatu thought impossible of humanity. This reunification via Harmonic Convergence could be what brings back the Air Benders, rather than Harmonic Convergence itself. Meanwhile, Asami, with the help of the newly acquired Varrick Global Industries, begins to take advantage of both the influx of new airbenders, as well as free access to the spirit world, to begin plans to mass produce more potent bending serums for use for the elites of society by capturing and experimenting on even more spirits, setting up an audience with the Earth Queen on the sale of these serums for military use.
Adding on to this I actually don't mind the idea of bending coming from lion turtles, themselves being physical embodiments of the elements they represent. There's no reason they couldn't have found a way to bend the energy these elements came from with various animals (badgermoles, skybison, etc) being the original recipients of these gifts. I like the idea of early humans taking sanctuary atop lion turtles and climbing down from them as their intelligence was allowed to develop without threat from natural predators, with the lion turtles imparting bending upon them as a sort of gift for their symbiosis. But yeah, I honestly like the idea of bending as an allegory for wealth and wished they leaned more into that premise as others have pointed out.