More of an introduction to a concept I thought of than an actual one-shot, but whatever.
Twin Avatars
She spread her feet, falling into an earth-bending stance before jerking both arms up in a block, a slab of rock rising to protect her. The flames slammed into it, and she held her stance, reinforcing the rock. She shifted, making grabbing motions as he tried to wrestle control of the stone from her. They silently struggled for a few moments before the rock crumbled under the pressure, and she registered a burst of flame. She dodged, using the tried and true air-bending techniques, before sending a compressed blast of air at her foe. He – naturally – dodged as well, before stomping the ground, causing a rock to leap up from the ground before careening towards her at his punch. She split it down the middle with her bending, along with the blast of flame that followed, before launching her counterattack. He dodged the fireball and called out tauntingly. "What's the matter, sister? Can't handle me?" Before sending a stream of rocks bursting from the ground, followed by a sweeping leg movement that propagated as a blade of flames.
"Why must we fight like this?" She called out, tears falling from the corners of her eyes. "We grew up together! We've known each other all our lives! I- I loved you! And I thought you loved me too!"
A shadow passed over his face, but he didn't let up his assault. For every burst of flame or air she dodged, she was forced to split or redirect a boulder. Talking didn't even slow him down. "I did. But you know why we can't be like that, sister. We're the two Avatars- I of Vaatu, and you of Raava. And as our predecessors did, we must fight until one of us is consumed by the other."
"What predecessors? We are the first twin Avatars born- There were none before us! Unalaq unleashed Vaatu and merged with him, but he wasn't born his Avatar!" With increasing desperation, she made a grand sweep with her leg, creating a twister of flame to knock away his attacks. It seemed to give him pause- a reminder that she too held great power. "We don't have to fight- we could be together. The past Avatars were not Raava- they made their own choices, for good or ill. Keeping the balance is a duty, but one must choose to follow it. People expect of me to keep the balance- but no-one expects you to disrupt it. Please don't so this." She was begging now, tears streaming down her face. "Please…"
"He won't stop whispering in my ear…" He murmured, and she felt hope- he didn't want to do this. "He's always whispering in my dreams. 'Kill her. Kill Raava's Avatar.'" He looked at her, despair written across his features. "I can't fight him."
"Yes you can." She told him, her voice no longer wavering, growing stronger and solid, like the rock she bent. "Raava has no power over me- I make my choices. You can do the same." She smiled at him. "She's been calling to me all along as well; 'Grab Vaatu. Don't let him go. Don't let him reign free.' I think that's what drew us together, why we first talked to each other. But we don't have to let them dictate our lives- We make our own choices."
She could see hope blossoming on his face, and she knew it would be alright when he gave her that impish smirk. "But what will people think when we refer to each other as siblings? I don't think they'll approve."
She giggled. "I'm pretty sure its common knowledge we aren't actually related. And we could just stop calling each other that."
Twin Avatars
Raava said that if she or Vaatu were destroyed, they would be reborn from the other. I figured that since no human can live their life without feeling dark or light at any point in their life, Raava's bond with her Avatar would mean Vaatu would be reborn very quickly- most likely enough for every Avatar cycle. So I figured it might be possible for Vaatu to have started his own Avatar cycle, being reborn in a human at around the same time as Raava is so that there are two Avatar's at any given time.
While Unalaq could only water bend due to Vaatu not 'holding the other elements', I figured Vaatu would gain it when melded with Raava, so future 'dark' Avatars would have the ability. While we don't know the exact details of an Avatar's birth, I like the idea of Raava's Avatar having the possibility of being dark (I figure such an Avatar would not have access to the Avatar state and Raava's full might) so I figured the same would go for a 'dark' Avatar being good.
To reiterate- the two unnamed Avatar's are calling each other 'brother' and 'sister' because they are Avatars, not because of actual familial relationship. And this female Avatar is Korra's successor.
I also figured Raava's and Vaatu's Avatars would always be of opposite genders, but not always a female Raavatar and male Avaatu.