The basic framework I've worked with is an AU.
It's a Sozin's Comet AU where Azula wins her fight against Zuko, and kills Katara, too. Meanwhile, Ozai either does or doesn't win his fight against Aang; I go back and forth. What's important is that Aang dies. So, whether Ozai lives or dies, the Fire Nation is totally victorious.
But Azula's still fucking crazy. And she's still, at least nominally, the Fire Lord. So what I work with is the idea that Azula, in her madness, starts to get paranoid about the nobles and the palace servants "depriving" her. Not granting her the "bounty" and the "privilege" that she's due as Fire Lord. So she starts demanding excesses of things. In particular, she starts demanding ever-increasing amounts of food, only the richest and most decadent delicacies.
This naturally results in her getting fat as fuck. Fast forward a few years, her weight has exploded and her deteriorated mental state, which never gets tended to because no one dares bring it up, has made her an immense, revolting blob of lard. She regularly doesn't bathe because she's afraid she's going to be drowned when she's cleaned. She belches and farts openly because she thinks it's her "royal right."
At the absolute maximum edge of depravity I like to fantasize about her indulging in cannibalism. Kind of in the spirit of what the Skekses do in The Dark Crystal: Age of Resistance. She believes the Fire Nation's worldwide empire deserves to be "harvested" for her benefit. So she starts demanding that cities and towns in the conquered territories send her tributes of men and women, which she horrifically has cooked, and which she eats. All the while being an enormous whale of fat that reeks and stinks.
Like I said, extremely nasty and perverted. I'm fucked up, I know it.