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.
They could start as strangers who are initially suspicious of each other. For example, when Ben first arrives in the Avatar world, Aang might think he's a new kind of threat. But as they learn more about each other's powers and intentions, they begin to trust and cooperate. Ben could use his alien forms like Heatblast to help Aang in a fight against a fire - bending enemy, while Aang could use his air - bending to give Ben an advantage in aerial combat.
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.
Well, a really interesting plot could be a post - war scenario where Aang and Azula are forced to interact due to some political unrest. Aang wants to bring peace and Azula, with her strategic mind, could either be an obstacle or an unexpected ally. Or perhaps a fanfic set in an alternate universe where Aang and Azula are raised together. Their different bending abilities would create a unique dynamic, and we could explore how their friendship or rivalry develops.
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.
Perhaps Ben, while exploring a new planet in his travels, stumbles upon a world that has a very Avatar - like culture and power system. He has to learn to use their ways in combination with his alien forms to survive. For example, using Heatblast's heat powers in a more refined way similar to firebending.
One great Aang fanfic could be one where he has to face an unexpected new threat in the Spirit World. It might explore his growth as the Avatar even further, showing his unique connection to the spirits and how he uses airbending in creative ways to overcome challenges.
One plot idea could be a story where Aang and Katara have to face a new threat together that specifically targets their relationship. Maybe a dark spirit tries to create misunderstandings between them by making Aang see false visions of Katara being unfaithful and Katara seeing Aang as being too controlling. They then have to work through these false impressions and reaffirm their love for each other.
They are popular because Aang is such a beloved character. His death in a fanfic creates a lot of drama and allows for exploration of different characters' emotions. Fans like to see how the other characters would react and grow in the face of such a great loss.
One great Aang fanfic is 'Aang's New Adventure'. It takes place right after the end of the series. In this fanfic, Aang has to deal with some leftover Fire Nation rebels who don't want to accept the new peace. It's really exciting as it shows Aang using his bending skills in new and creative ways to bring about peace once again.
Their relationship could develop through a series of challenges. For example, they could be forced to team up against a common enemy. This would make them start to respect each other's abilities.