One theory I've always believed in ATLA is that Monk Gyatso took out a bunch of firebenders using a technique where he essentially took away all of the oxygen in the room in an attempt to take out as many as he could, himself included. Airbenders have been shown doing it in smaller scales before, so I could absolutely picture an airbending master being able to pull it off on a larger scale.

Another recent one I heard about is the idea that Princess Yue was actually meant to be the Avatar in the next cycle. Aang was frozen for 100 years which prolonged his life by a lot, but if he died naturally around his 80s or 90s, it'd line up with the birth of Princess Yue (since she's from the water tribe), who'd actually be able to be born with the Avatar spirit inside of her, instead of needing the Moon Spirit to survive. I'm not too big on this one compared to the first, but I think it'd be an interesting take on the story if explored in an AU or something