I'm just a recent fan of the Avatar cartoons, and started watching years after the original series ended in '08. I know there are going to be alot of individuals saying this idea is stupid or that I'm trolling....nothing can be further from the truth. My idea and theory is actually based off a book written in the 70's by Frank Herbert, the creator of the infamous sci fi novels of DUNE. There are 6 original series, but the one that is most similar is called The Dune Messiah, in which the main character--Paul Atreides--is the chosen one that will save the entire known universe and set order. Now, his entire cult relgion is based off of all Paul's teachings and wisdom and basically more than dominates the universe, but literally controls everything and everyone around it, basically a supressive authoratative regime, ruled by one person. Now Paul, like Aang, are the supreme beings and perfect in every sense of the word, but unlike Paul, Aang's position was nothing more than just a peacemaker over the elements, but are alike since they both have followers/admirers and enemies alike. During the span of about 10 years, Paul become disillusioned with what he created--as how I feel how Aang must feel like with Liberty City becoming corrupt with benders abusing non benders. Paul survives an assasination attempt but is left blind--but still able to see. But according to the laws of Dune on planet Arrakis, being handicapped is a sign of weakness must be taken to the deep desert so that the giant worms can consume the disabled one. Eventhough everyone disagreed with the written laws and begged Paul not to go forth with his exile and eventual death, Paul still does and wanders the dessert, to be killed by the giant worms. But Paul survives, and 10 years later, a blind hermit form the desert starts talking about revolution and anti-establishment.....it is Paul, coming back to try to destroy what he had created 20 years before and discrediting his sister--the leader-- the entire prophet thing, and other problems that his cult caused.
This is the same way I think, Aang somehow split his spirit, releasing 3 of the 4 bending abilities and retaining his deafult skill--Air/Energybending......thus Korra can't airbend because that part is still being used by the still alive 85 year old Aang. It sounds weird, but also in Dune Messiah, Paul's son, was also a future messiah in the making and there were 2 of them living at the same time, while Paul was posing as the the Prophet. Upon the death of Paul, which he was killed eventually (forgot how), his son, Leto II, eventually became the God Emperor of Dune that was the supreme dictator for over 3,500 years. Since I believe Amon to be Aang, somehow, his eventual death will give Korra full avatar powers and becmoing the full Avatar she should be. Just like the prophet was anti-establishment and had faked his death in order to do this, Amon too, is anti establishment and anti bender due to their abuses agsint non benders. There is a corelation between both the book and series in this sense. Aang being one of the most spiritiual, can somehow and was somehow able to split himself and fake death, just as Paul did....only to return and go up against, that which they created.