A really interesting plot could be that Toph discovers she has some unique power related to her unborn child that has never been seen before in the Avatar universe. This could attract the attention of both friends and foes. There could also be a plot where Toph is pregnant during a time of unrest in the world, and she has to balance her pregnancy with her responsibilities to protect her people or help the Avatar. Or perhaps she goes on a journey to find a special place or object that is said to ensure a safe pregnancy and birth for her child.
One idea could be that they face challenges in a world not fully accepting of their relationship and impending parenthood. Their friends might have mixed reactions, and they have to work together to prove that their love and their new family can be strong. For example, Aang could be initially worried about how this would affect the balance in the world, but then come around to support them.
A possible plot could be that Toph and Zuko are forced into an arranged marriage by their families or kingdoms for political reasons. Then, Toph gets pregnant unexpectedly. This leads to them having to figure out their relationship on a deeper level. They start to bond over the pregnancy, sharing their hopes and fears for the future. Maybe they go on a journey to find a special place where they can raise their child in peace, away from the politics and power struggles that surround them.
One idea could be that they face challenges from their respective nations due to their relationship. Zuko's Fire Nation might be worried about the influence of a Water Tribe member, and Katara's tribe could have concerns about Zuko's past. Another plot could involve them preparing for the arrival of their child while also trying to bring peace between the Fire and Water Nations. For example, they could use their unborn child as a symbol of hope for a united future.
One plot idea could be that the OC is a new earthbender in town. Toph senses their potential and decides to train them. Along the way, they face challenges like a group of earthbenders who don't want an outsider learning their ways. Another could be the OC is from a different nation but has an affinity for earthbending. Toph helps them hide this from others while also teaching them. Or perhaps the OC has a special ability related to earthbending that Toph is intrigued by and they go on a journey to discover the origin of this ability.
One idea could be a story where Katara and Toph go on a journey to find a rare and powerful bending artifact. Along the way, they have to face various challenges like difficult terrains that Toph helps them navigate through her earth - bending, and dangerous water bodies where Katara's skills are crucial. They also have to deal with other benders who want the artifact for themselves.
She was from a wealthy family but she didn't conform to their expectations. Instead, she chose to follow her own path and become an earthbending master. She was also one of the first female earthbending masters in a long time.
A great avatar toph fanfic could focus on her relationship with other characters. Like a story where she forms a deep friendship with a non - bender. They go on a journey together and she has to protect them using her earthbending skills. This would show a different side of Toph, her loyalty and caring nature outside of just being a tough bender.
One idea could be a crossover where Pokemon trainers visit the Avatar world. They could learn bending techniques and use their Pokemon in unique ways to help the Avatar in a great battle. For example, a Pikachu could use its electricity to enhance a waterbender's attack.
One idea could be exploring how the pregnancy affects Neytiri's role in the clan. Maybe she has to balance her duties with her new physical state. For example, she could be training the younger Na'vi while also dealing with the challenges of pregnancy, like increased fatigue but still pushing through to be a strong leader and role model.
In a naruto x avatar fanfiction, there could be a scenario where the Akatsuki from Naruto's world teams up with some of the villains from the Avatar world. This would force Naruto and the Avatar (whichever one it may be) to put aside their differences and form an alliance with other characters from both worlds to stop this powerful and evil coalition.