I do not own Avatar. If I did, the first episode of season three would've been shown by now.
This is just another version of what'd happen if the new "Defenders" of our beloved fandom took control of canon. Just another theory...
Sarah did not know how or why she had wound up in a new world. All she knew was that the rabbaroo did not exist in her own world, so she had to be in some new place. She also knew that she was trapped in the rabbaroo's exhibit until morning, so she found a spot free of droppings and went to sleep.
Ba Sing Se was much more different than she had imagined it. She had an idea already after watching every episode of "Avatar" since "The City of Walls and Secrets", but now that she was in the city for real...
This is awesome, she thought, looking at the small pile of copper pieces the confused zoo keeper had given her as compensation for having to stay with the rabbaroo all night. I know where I want to spend some of these!
She got some food from a small shop and then dedicated the afternoon to searching for a certain tea shop. She finally found it in the early evening, opened the door... and...
WHAM!
"You're under arrest for treason!"
"What?" Sarah gasped. The Dai Li agents that had knocked her over pulled her roughly to her feet. Her hands were secured behind her back by their dreaded rock gloves. "What the heck did I do?"
"By inserting yourself into your favorite fandom and bending the will of the canon characters, you have commited high treason!" the Dai Li agent snapped.
"I didn't mean to come here and I never bent anyone's will!" Sarah cried, struggling against the rock gloves. "Let go of me!"
"Silence! Next time you'll think before you have sex with Prince Zuko!"
"I... WHAT?"
"WHAT did you just say?" came an enraged voice from inside the shop. "I never--"
"Don't interfere, boy!" the agent barked, looking inside the shop. "The last things you need to worry about are millions of little Fire Princes and/or Princesses running around the Earth Kingdom. In fact, the Overlords have ordered you castrated. This is for your own good, so I suggest you come quietly!"
If only Iroh had been there to hand their asses to them on a platter. Too bad he was busy...
"Who's bending whose will now?" Sarah demanded as Zuko was taken into custody.
And so Zuko and Sarah met under the worst possible circumstances.
After being "fixed", Zuko was released back into the wilds of the Lower Ring of Ba Sing Se. He never had the balls to sneak into the secret brainwashing facility to steal Appa.
Instead, Appa was returned to Aang and the gang by the new Overlords of the City and Fandom, who had taken down Long Feng quite a long time ago. In this way, the dictators gained the trust of the characters.
The Overlords, however, surprised everyone by practically handing the Earth King to Ozai's Angels when the time came. According to them, everything was the way it was meant to be, and they went back to their own world to enjoy some "quality fanfic".
However, Zuko, who still lacked balls, cowered when it came time to face Azula. He hid behind some crystals and sucked his thumb when Aang came to rescue Katara, and Iroh him. Iroh went to comfort Zuko, which took longer than he thought.
And Azula, who had no restraint or compassion for her enemies, fried both Aang and Katara as soon as the Dai Li arrived to save her cowardly, little buns. The world's last hope for peace and the one he loved were gone forever, but there was much rejoicing among fans of the show when it was realized that all fighting over who Katara should marry would end. The only ones who wept were the ones who had been banished from the fandom.
Zuko and Iroh were shipped back to the Fire Nation and executed. Zuko, who STILL lacked balls, kept a freaked-out sort of expression until the very end.
Sokka, Toph, the Earth King, and Bosco escaped with Appa and Momo, but the last sky bison and winged lemur were eaten by predators. Bosco was wounded in the fight and died a week later. The grieving Earth King hurled himself off a cliff the next day. Sokka and Toph were tracked down and slaughtered by the Fire Nation before they could reach the safety of God-Knows-Where.
As for Sarah, the methods which the Overlords used to deal with helpless fourteen-year-olds were never known. We can safely assume, however, that she was brutally killed before Ozai's Angels came along.
At least none of them had to see the terrible yet useless power that Sozin's Comet gave the Fire Nation and her new allies, the indirect destroyers of Ba Sing Se.
Every time you send a flame, the vet reminds Iroh that Zuko should be "fixed", which is quite uncomfortable for them both. Please. Think of Prince Zuko.