Originally Posted by
Arrowstorm
The flashback, as first sign. It shows a bender who uses bending to spread terror and dominance. What if that wasn't the last bender who were a total douche? What if Aang realized that all the threats to the people are benders, and that there will never be peace and equality between people before nobody are superior benders? Maybe, after all these years, he decided to fake his own death, and take his own bending away, which made the Avatar cycle continue, but let him still keep the power to remove bending?
Maybe Korra hasn't been able to enter the Avatar Mode yet because Aang still living is cutting her off from being a full avatar yet? And being an old man hasn't stopped anybody in the series from being a total badass ever.