Sokka would be protective of his sister. He would probably start by trying to negotiate with Zuko's side to call off the marriage. If that doesn't work, he might come up with a plan to rescue Katara, perhaps enlisting the help of their other friends and allies.
Aang would be very happy and supportive. He has always been a friend to both Zuko and Katara, and he would see their union and the upcoming baby as a positive step towards more harmony in the world. Sokka might be a bit overprotective at first, especially towards Katara. But eventually, he would come around and be excited about being an uncle. Toph would probably make some sarcastic but also very endearing comments about the whole situation.
Some of their friends from the Fire Nation, like Mai, could be hesitant. She might worry about the implications for the Fire Nation's future and Zuko's role as a leader. However, as time passes and she sees how much Zuko loves Katara and their unborn child, she becomes more accepting. On the other hand, the Water Tribe elders could have mixed reactions. Some might see it as a good sign of unity, while others might be concerned about cultural differences.
It would likely become more complex. They would have to rely on each other more as parents. Their love might deepen as they share the experience of raising a child together.
One possible plot development could be Katara initially resisting the marriage wholeheartedly. She might try to escape or find a way to nullify the forced union. This could lead to a lot of tension between her and Zuko, as he may be more accepting of the situation due to political or family reasons.
Their relationship would likely become more intimate and complex. They would have to rely on each other more, sharing a bond that goes beyond their previous friendship or alliance. Zuko would probably become more protective of Katara, and Katara would trust Zuko with her vulnerability during pregnancy.
In the fanfiction, the baby might have a unique appearance. It could have Katara's blue eyes and Zuko's dark hair. Personality - wise, it could be a blend of their traits. Zuko's determination and Katara's compassion. Maybe the baby is also very empathetic, able to sense the emotions of others around it, which is a trait that could come from both of their sensitive natures.
Some fanfictions explore their role in rebuilding the world after the war. Zuko and Katara could be working together to restore balance between the nations. This kind of fanfiction often shows their leadership skills and how they complement each other.
Aang would feel a sense of urgency and his connection to Katara would make him even more determined. He would call upon the spirits if necessary to find her. Sokka would be full of fury and his strategic mind would be working overtime to come up with a plan. He would also be very protective of his sister's honor during this time. Toph would be steadfast in her support for the rescue. She might even create some distractions to help the others get closer to Katara. The Fire Nation soldiers who respect Zuko might be confused by his actions, while those who dislike him might see it as an opportunity to bring him down. And Katara would be using her waterbending skills to defend herself and try to escape at every opportunity.
Sokka would probably be overprotective. He's always been protective of Katara, and now with her being pregnant, he would be constantly worried about her safety and the well - being of the baby. He might offer all kinds of advice, some of which might be a bit silly but well - intentioned.
Katara could be heartbroken. She might initially be in denial, not believing that Aang has actually left. Then she could become angry, feeling betrayed by Aang's sudden departure.