One idea could be exploring the relationship between the new characters introduced in Avatar 2 in more depth. For example, a fanfic could focus on the friendship or rivalry between two young Na'vi from different clans. Another plot could revolve around a human character who becomes stranded on Pandora and has to learn to survive with the help of the Na'vi. They could discover new areas of Pandora that were not shown in the movie, filled with unique flora and fauna. A third idea might be a fanfic that delves into the backstory of one of the main Avatar 2 characters, perhaps their family history or how they came to be in their current position.
One idea could be a crossover where Ben 10's Omnitrix malfunctions near Avatar Aang's world. Ben and Aang team up to face a new threat that is causing chaos in both their realms. Maybe some of Ben's alien forms have unexpected interactions with the bending abilities in the Avatar world.
In a fanfiction, the Avatar State of Aang could be used in a more creative way. For example, Aang accidentally enters the Avatar State during a cultural festival. His friends are initially worried, but then they realize that in this state, Aang can communicate with the spirits present at the festival in a unique way. He learns secrets about the spirits and their connection to the human world, which he then uses to strengthen the bond between the two realms when he's out of the Avatar State.
Some might focus on the training of an Avatar's feet. Since the Avatar has to master different bending forms, their feet play a crucial role. In a fanfic, it could be about an Avatar's struggle to perfect the stances that require precise footwork in firebending, like the quick and powerful movements. Or in airbending, where the feet need to be light and flowing. This could also tie in with a mentor - apprentice relationship, where the mentor corrects the Avatar's foot positions to help them improve their bending skills.
One idea could be a story where the two teams compete in a friendly but intense tournament. They might face challenges that test their bending skills in new ways.
Another fanfiction could explore the challenges they face during Katara's pregnancy. Since Aang is the Avatar, he has many responsibilities. There could be a conflict where he has to leave for an important mission, but he doesn't want to leave Katara alone. Katara, being strong - willed, tells him to go and fulfill his duties. In his absence, she discovers new water - bending techniques related to pregnancy and childbirth, which she plans to teach other women in their community later.
A good plot idea could be about an Avatar who loses their bending abilities. They then have to go on a journey to regain them, facing various challenges along the way. Maybe they have to seek the help of a reclusive and powerful spirit who demands difficult tasks in return. Another idea is a story set during a time of peace in the Avatar world, but a new threat emerges from an unexpected source, like a group of non - benders who have found a way to harness dark energy. Or you could do a plot where the Avatar has to deal with a civil war within one of the bending nations, trying to bring balance and peace back.
One idea could be exploring the backstory of a minor character from the Avatar world in the sequel fanfic. For example, a Na'vi who was on the fringes of the main events but has their own interesting journey. Another could be a story about the first human - Na'vi hybrid, dealing with identity issues and being a bridge between the two cultures. Or perhaps a fanfic that focuses on the restoration of a part of Pandora that was damaged during the events of the movie, showing how the Na'vi and any sympathetic humans work together to heal the land.
One idea could be a crossover where Naruto characters are able to bend elements like in Avatar. For example, Naruto discovers he has the ability to airbend. This could lead to a whole new adventure as he tries to master this new skill while also dealing with his ninja duties in the Hidden Leaf Village.
One idea could be to explore the backstories of the avatars in the brotherhood. Maybe they all come from different worlds or cultures and how they came together to form the brotherhood. Another could be a story about a new avatar joining the brotherhood and facing challenges to be accepted.