One interesting plot twist could be that the kidnappers turn out to be working for an old enemy of Zuko's family. They initially thought it was just a random act of villainy, but as the story unfolds, they discover a deeper conspiracy aimed at destabilizing the Fire Nation. Another twist might be that Katara discovers she has a hidden power that she can use to help them escape, but using it comes with a great cost. Maybe it drains her life force or has some other negative consequence.
A possible plot is that they discover some unique power or ability in their unborn child. This leads to various groups either trying to protect or capture them. They have to go on a journey to keep their baby safe. Also, they could struggle with how to raise a child with such a special gift in a world that is still recovering from war.
A possible plot twist is that the kidnapper is being controlled by an otherworldly force, and Mokuba discovers a way to break the control. For example, he might find a magical item within the kidnapping location. Another plot twist could be that Mokuba's kidnapping was part of a bigger scheme to frame Seto Kaiba for a crime he didn't commit. This would add a layer of drama as Seto has to race against time to clear his name and save his brother. Also, it could be that the kidnapper is actually Mokuba's long - lost relative who wants to reconnect in a very extreme way.
Maybe the Akatsuki members who kidnapped Naruto start to have second thoughts. Some of them might realize that Naruto is not just a tool for their goals. They could begin to see his value as a person and his potential to bring peace in a different way. This could lead to internal conflicts within the Akatsuki group.
A really unexpected plot twist could be that the kidnappers are actually the good guys in a convoluted way. Maybe they know of a prophecy that Sasuke and Sakura are fated to bring destruction, so they are trying to stop it. Another idea is that during the kidnapping, Sasuke and Sakura end up in a different dimension where they have to rely on each other in completely new ways. Plus, the motives of the kidnappers could be related to a secret that Sakura has been hiding about her family all along.
Well, perhaps Katara is kidnapped by a bounty hunter who has been hired by someone who has a vendetta against the Avatar's team. The kidnapper could be taking her to a far - off and desolate location. Then, Aang and the others have to track her down through various difficult terrains. Or, it could be that she's kidnapped by a group of dark benders who want to study her waterbending to find a way to corrupt it. They might keep her in an underground lair filled with all sorts of dark magic - like contraptions.
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.
One interesting plot development could be that through training with the dragons, Zuko and Katara start to understand each other's cultures better. As they learn the dragon - inspired bending techniques, they see how Fire Nation and Water Tribe elements can blend in unexpected ways. For example, Zuko might use water - like fluidity in his firebending, inspired by Katara's waterbending style. And Katara could incorporate the intense heat and energy of fire into her ice - making, creating a new form of waterbending that is more powerful.
One possible plot twist could be that the kidnappers are not who they seem. Maybe they are being controlled by an ancient evil force that wants to use Legolas for some dark ritual. Another twist could be that Legolas is kidnapped as a decoy to draw out his friends, and then the real target is revealed later. For example, the kidnappers might be after a powerful elven artifact that Legolas's friends possess.
One possible plot twist could be that the kidnappers turn out to be working for an old enemy of the Hidden Leaf who has a personal vendetta against Naruto. Maybe they plan to use Naruto as a sacrifice to revive an ancient evil. Another twist could be that the kidnappers are actually being controlled by a powerful genjutsu user within the village who wants to disrupt the sealing for their own political gain.
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.