The best twists in these stories are the ones that reconfigure the setting’s mechanics. I get tired of rewrites where Aang learns faster or Zuko switches sides earlier. The memorable ones totally reroute the premise. One I love has the spirit world becoming actively hostile, not a neutral place, because the 100-year war has corrupted it, and the Gaang has to somehow mediate peace with the spirits while still fighting the Fire Nation. That’s a whole different type of balance. Then there are the ones that mess with the Avatar cycle itself. I read one where Aang is actually the last Avatar, period—the cycle was broken by Roku’s death, and he only has access to his own past lives, not to a future. It creates this terrifying pressure. The unique ones aren’t about “what if” with the canon events but “what if” with the rules of the universe. Those are the stories that stay with me.
Another fun twist I see sometimes is a role reversal in the elemental masters. Like, what if the Air Nomads weren't wiped out, but instead went into hiding and became a militant, isolationist society, and Aang has to reconcile that? It flips the entire moral foundation his culture gives him. Or fics where waterbenders are the imperialist aggressors instead of firebenders. Those kinds of systemic flips force the characters to navigate a completely different ideological landscape, which is way more interesting than just changing a romance pairing.
The personal identity twists can be brutal, too. I remember one where Zuko discovers he's actually Ozai's nephew, not his son, because of some palace intrigue, and his whole quest for honor was based on a lie from the start. That sort of psychological unraveling hits harder than any grand battle scene.