One common theme is Tenzin learning from Aang. Since Aang was Tenzin's father and also a great Avatar, many fanfictions explore how Tenzin grew under Aang's guidance. For example, Aang teaching Tenzin about airbending techniques or the spiritual side of being an airbender.
Family relationships are also a big theme. Fanfictions often depict the family dynamic between Aang, Tenzin, and the rest of the family. It could be about how Tenzin coped with the pressure of being Aang's son, or how Aang tried to balance his role as a father and the Avatar. Some stories might also include the siblings of Tenzin and how Aang interacted with all of them.
One common theme could be their relationship dynamics. Since they have a complex relationship in the original story, fanfiction might explore different aspects of it, like their past misunderstandings or how they might reconcile.
One common theme might be the exploration of the characters' growth. Lin and Tenzin would have to adapt to the changes in their relationship and their own lives. There could also be themes related to the support system around them, like how their friends and family react to the news of the pregnancy.
Friendship is a big theme too. Their bond can be explored, how they support each other through tough times, and how they grow as friends. It could involve them having deep conversations, or just having fun times together like exploring new places in the Avatar realm.
Fanfiction.net is also a great place to look. Just type in 'Aang and Tenzin' and browse through the results. However, the quality may vary, so it's good to read the reviews or summaries first. Some stories might focus on their relationship as teacher - student, while others could explore their family bond in more creative ways.
Another theme might be his loneliness. Aang was the last airbender for a long time. He may feel isolated even among his friends because they can't fully understand what it's like to be the sole survivor of his nation. This sense of loneliness can contribute to his depression in fanfiction.
A common theme is the exploration of their age - gap relationship. Tenzin is much older, so there's often a focus on how they bridge that gap. Another theme is the clash between Tenzin's traditional airbending wisdom and Korra's more impulsive Avatar ways.
One common theme is friendship. Since they come from different backgrounds in the Avatar universe, exploring their growing friendship can be interesting. Another theme could be adventure, as they might go on quests or face challenges together. Also, there could be a theme of cultural exchange, with Aang being an Air Nomad and Onji being from the Earth Kingdom.
One common theme is self - discovery. Aang often has to look within himself to find the path to enlightenment. Another theme is the connection to the spirit world. Since Aang is the Avatar, his enlightenment is closely tied to his understanding of the spirit world.
One common theme is the corruption of Aang's pure heart. Since he was the Avatar, a symbol of peace and balance, seeing him corrupted by dark forces creates a strong contrast. Another theme is the struggle between light and dark within him. He has to fight his own inner demons.
One common theme could be self - discovery. Aang having hair might be seen as a new part of his identity, and fanfictions could explore how he comes to terms with this change. For example, he might have to deal with how others perceive him differently.