Disclaimer: Avatar does not belong to me!
AUTHOR'S NOTE: This takes place in the Spirit World, so canon characters and the timeline are somwhat warped. Not utterly mangled beyond discription, but be prepared for weirdness.
"It is your choice now."
It was Teni, spirit of stone, who spoke, as her brother, Luru, spirit of earth, watched the last of life flee from Avatar Kyoshi's body.
The people to whom she spoke were Taza, spirit of lightning, and Agni, spirit of fire.
"Who will you choose?"
Taza's hot golden eyes scanned the mortal world, looking for a hopeful candidate. Her sight catches on the pregnant Fire Nation woman, and she knows this is the one.
She touches Agni's arm and points, drawing his bright, flame-colored eyes to the woman.
"This is the one?"
"Yes."
La and Twi glance at each other, and do not comment. They live in the mortal world, but they must be here for and event as important as the choosing of the next Avatar.
Niu, spirit of wind, says, in her whispery, pale voice "She does not look strong."
Gui, the spirit of air, agrees. "Do you know if she will bear the next one?"
Taza looks sharply at the pair. "Yes."
Luru speaks, for the first time, his voice deep, and ever calm. "Ona should be here."
"She is greeting Kyoshi." Teni reminds him.
"Yes. But she is always here for the choosing. We should wait."
"You are the very essance of waiting, Luru. You wait for everything." said Teni irritably.
Just then, Ona appeared. She was a striking woman, with long silver hair pulled sharply away from her face.
"First Avatar." murmured Agni.
Ona smiled slightly at him. "It is your turn, is it not?"
"Yes, Ona." replied Taza. "We want this one." she pointed again to the woman.
Ona looked.
"A fine choice." she said to herself.
"But I do not know if he will be as great as Kyoshi."
Agni and Taza glanced at Ona, before nodding to each other.
"We choose her."
"What will his name be?" inquired Niu.
Taza stared at the as-of-yet unborn Avatar's mother, and said "Roku."
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READ THIS IF YOU ARE CONFUSED:
Does this need explaining?
Ok.
This story is based in the Spirit World, so all of the canon characters are kind of off in the distance. The story starts with before the war, a bit during Avatar Roku's time, through Aang's, and possibly an epilouge involving the birth of the next Avatar.
Character sheet:
The spirits introduced are the big ones; we already know about Twi and La, so there's no need to explain them(yet).
Agni is a semi-canon spirit of fire whose name was ripped off from the Agni Kai. He's kind of a sun god type guy.
Taza is the spirit of lightning. She's Agni's sister. Both Agni and Taza(as well as the rest of the spirits) have two forms, one animal and one human. Their animal is a dragon.
Niu is the spirit of wind, and Gui is her husband(for lack of a better term), and the spirit of air. Gui and Niu are different, even though it may not seem like it. I'm still trying to work that one out myself. Their form is actually a bird, not a Sky Bison.
Luru is the spirit of earth(as in dirt, soil, mud, ect.). Teni is his sister, and the spirit of stone. Their animal form is a badgermole.
Ona is the very first Avatar, and is special because she was basically the one who got life as we know it in the Avatarverse going. The other reason she's a real spirit and not just another dead Avatar is because she is, essentially, the Avatar State in human form. She was kind of like the omnipotent spirit on top of the mountain, but the big spirits changed their minds and decided on the reincarnation thing instead.
As the story goes on, there will probably be more minor spirits involved.
These guys are pretty old, and really powerful, so please expect some ridiculous shows of Mary-Sue-ish powers(but at least they're not the second Avatar or something...).