This is absolute total crack. There is no way it would ever be possible in the series.
Disclaimer: I own nothing, except the Ninja Juice
It was late one night in the Fire Nation palace when Prince Ozai, the future Firelord, roused his children from their rooms. Azula, his favorite, was exactly two years old where Zuko, the oldest, was currently four. Both of his children blinked up at him with tired eyes, but Ozai didn't care. He needed to do this tonight before anyone could catch him.
Ozai pulled a large bottle filled with black liquid from the pocket of his robes. "Children, don't tell your mother," he began, a sinister smile on his malevolent face, though his children were so used to looks like that they didn't notice the evilness lurking in their father's expression, "but this is something I have had the Fire Nation's best scientists make for you."
"Wha' is it?" Azula asked, her young voice intrigued. Both she and Zuko stared at the bottle with interest. It didn't look like anything they had ever seen before, but even in their young, childish minds that were forced to grow up way too fast, they knew that made sense since their father said he had the Fire Nation scientists make it. They just didn't know why he would.
"I call it Ninja Juice," he said, his grin widening even more as he anticipated the changes the Ninja Juice would cause in his children. "You're going to drink it and then you will become the best warriors the Fire Nation will ever see, especially you, Azula."
"Ninja juice will give you many interesting abilities: stealthy skills, the ability to track down people easier than you can tie your own shoes, the ability to survive practically anything, including getting slammed in the chest by giant rocks that should rupture your organs or landing on your necks during Agni Kais, and many other ninja-like abilities that will make you utterly frightening."
Both his children's eyes lit up and they looked eager to drink the ninja juice, which Ozai found pleasing.
"However," he said in a foreboding voice, "it has side effects that include: the obsessive need to take revenge, the inability to ever give up, extreme anger issues, inner turmoils that you may never overcome, the compulsive desire to control people with fear, sociopathic and psychopathic tendencies, and possible psychotic breakdowns."
Zuko and Azula now stared at the Ninja Juice with frightened expressions, but Ozai didn't notice.
"As soon as you drink, you will forget we ever had this conversation," he said, pulling the cap off the Ninja Juice. "So, who's first?"
AN: I came up with this during a conversation with my sister, but honestly, I have no idea what messed up part of my mind brought it forth. We were just talking about why Zuko and Azula always seemed to be practically indestructible and have such creepily cool abilities. I mean, Zuko gets hit with a rock from an earth bender twice: once in Zuko Alone and that other time when he was trying to join the Gaang and accidentally burned Toph's feet. Yet when it happens to Jet, Jet dies. And then Azula totally landed on her neck when she fell during that Agni Kai. So... we thought it'd be funny if the Firelord gave them Ninja Juice and I decided to write it even though I know it is completely impossible and this is nothing but a parody/crack.
-DragonsintheMoonlight :)