The plot bunny instilled in me this idea that won't go away, so enjoy!
Prologue: (Let's pretend everyone is a year older) 7 years ago.
Ozai stood at the foot of the Eastern Air Temple.
"Love, you know that this is the last thing I wanted to do. I love you with all my heart and soul."
A tear fell from Ursa's cheek.
"I had to do it. I couldn't let you just kill my son. Besides, you got what you wanted. Your wish to be Fire Lord has been granted." She tried her hardest to not burst out crying.
"Being Fire Lord is an empty victory without you, but you would hunted down if I let you live in the Fire Nation."
"I know the price of what I did, and it's a price I'm willing to pay for my son."
"I know, and I'm sorry." Ozai leaned in closer to Ursa and left a soft kiss upon her lips.
"Ozai, please don't..."
"But I love you..."
"I do too, so much, but you're making it harder than it has to be."
"Actually you are. Think of it as a final goodbye"
The moon fell upon the two lovers.
(9 months later)
Ursa held her newborn daughter against her breast.
"The horrors of the world won't be a bother to us. We will live here and you will never know any of the horrors of the world as long as we both shall live. Aimi, it is just you and me now."
(4 years later)
Ursa's messenger hawk sent her a report that her son Zuko had just been banished from the Fire Nation. She fell to her knees and began to cry. Aimi ran towards her mother.
"Mama, what's wrong. Why are you crying?" The young girl had never seen her mother cry, and she hardly ever cried herself, so this was all a bit of a shock.
"Honey, I believe that it is time that we left the eastern air temple, we're never going to escape this war, no matter where we go."
Ursa flipped the scroll over and wrote to her husband to send a hot air balloon to the eastern air temple, and to send them to the Earth Kingdom.
(Several months later)
Aimi saw a giant red balloon in the sky.
"Mama! Mama! Is that the balloon that's going to take us off this place?" Aimi stared at the balloon intently. It was very new to her.
Ursa was further into the temple. What her husband didn't know was that she too was a firebender. Aimi was probably a firebender as well, but she tried to keep the Fire Nation away from her daughter as much as possible. She burned all signs of recent life from the Eastern Air Temple. No one would ever know that she was here. Then she heard her daughter.
"Yes honey that's it. I'm coming."
(A week or 2 later)
Ursa stood in the slave sales in Gaoling. Aimi stood beside her, clutching her hand tightly.
"What's going to happen to us Mama?" Aimi looked up at her mother, frightened.
"I don't know." Ursa tried her hardest not to cry. That's when she noticed that her daughter had just been sold to a Chang family.
"NO!! Please don't take my daughter!" Ursa burst out sobbing for her daughter. She ran out to grab her and placed a scroll in her hand. "AIMI!!"
"Um, how much for the sobbing woman?" A man with a moustache and fancy robes called out. Ursa stared at him.
"100 gold pieces."
"Sold."
The slave seller took Ursa to her new owner.
"I am Mr. Bei Fong. I am a neighbor to the Changs. You will be able to see your daughter again." As he whispered into her ear, Ursa's tears came to a stop.
(4 years earlier)
Ursa sat by the fountain with a scroll in one hand and her daughter asleep in the other
Dearest Daughter,
If you are reading this, it means that we are separated and you know that it's time to look for your brother. You are not who you think you are. You are a princess, a princess of the Fire Nation. I was the former Princess Ursa. Your father is Fire Lord Ozai. But I was banished for something and that is how we came to live at the Eastern Air Temple. I love you so much, and I hope that you will find a way to stop the war, and find your brother, Zuko.
Your Beloved Mother,
Ursa
Ursa closed that letter and promised to give it to her daughter when the time was right.
(7 years later) (Present Day)
7 year old Aimi stood outside her window in her extravagant room in the upper ring of Ba Sing Se. 3 years ago, the Shuns had adopted her from the Changs and she moved to Ba Sing Se. She never knew that she was from the Fire Nation, much the less a firebender, but oftentimes she would accidentally set her curtains on fire, and she didn't know why.
Today was the day that would change her forever.
"Lady Shun, there is a letter for you." A servant held out a scroll for her.
"Thank you, Izu." Aimi took the scroll from her and opened it.
Daughter,
How I have missed you so! The Bei Fongs oftentimes tell me of how much you are growing. Work here has been hard since their daughter disappeared. I believe you met her a year ago. The blind one, remember?
But this is not why I write to you. You should still have the scroll I gave to you when you were sold to the Changs. It is time to read it.
Aimi had hardly even remembered the scroll, but she looked in her secret compartment where she kept all her old possessions. True to her mother's word, it was there. She opened it and read.
As she finished the old scroll she shook her head in disbelief. She picked up the letter she had just gotten
You have just read it. If you listen to the politics that your adoptive parents talk about, you should know that the Fire Nation is the enemy. Zuko is your brother who is ten years older than you. From sources I have heard that he betrayed your father and went off help the Avatar.
Daughter, I need you to go back to the Eastern Air Temple and help your brother defeat your father. I know you must be thinking that a seven year old can't do this, but when you were born, I had a vision of me birthing a dragon, and I knew that it meant that you had the spirit of the first firebenders in you and that you weilded great power. It is time for you to learn.
Ursa.
Aimi understood. The night earlier, she had had a dream of her turning into a dragon, and suddenly it all made sense.
The next morning, the Shuns woke up to sound of servants yelling that Aimi had disappeared.
So there you go.