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.
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.
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.
Yes, there are quite a few interesting 'Avatar Aang and Katara' fanfictions out there. You can start by looking on popular fanfiction websites like Archive of Our Own. Some fanfictions explore their relationship further after the events of the show, delving into their daily lives and how they grow together.
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.
Well, physically, a female Aang might have different hairstyles and clothing. Instead of Aang's simple monk robes, she could have a more feminine take on them, like with added ribbons or a different cut. Her hair could be longer and styled in ways that are more typical of female characters in the Avatar world. In terms of her bending style, it might be more graceful. While Aang was fluid, a female Aang could bring an extra level of elegance to her airbending moves.
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.
One common theme is power control. Aang often has to learn how to handle the immense power of the Avatar State without losing himself. For example, in many fanfictions, he struggles with the sudden influx of energy and the potential for destruction it holds.