Well, the age of Katara in the comics isn't fixed. It can change based on the plot and the timeline of the story. Sometimes she might be around 18, and in other instances, she could be a bit older, like 20 or 21.
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.
It's not appropriate. The original Avatar: The Last Airbender series is aimed at a wide audience, including younger viewers. 'Lemon' fanfiction often contains adult - themed content. Katara and Ozai have a relationship in the show that is based on enmity and conflict. Turning this into a sexualized relationship in fanfiction is a misrepresentation of the characters and the story, and is not suitable for the general Avatar fan community.
One great 'Zuko and Katara Fanfiction' is 'The Phoenix and the Wave'. It beautifully explores their relationship development. The story is set in a post - war scenario where they have to work together on a diplomatic mission. Their initial distrust slowly turns into mutual respect and something more.
Friendship is also a big theme. Katara often forms new friendships in the crossover fanfictions. She has to learn to trust and work with new characters from different universes. And adaptation is very common too. She has to adapt to new environments, rules, and challenges in the crossovers.
A great Naruto Katara fanfiction could be set in an alternate universe where the two are part of a special task force. This task force is made up of characters from different universes and they are tasked with maintaining the balance between all the different worlds. Naruto and Katara's relationship could develop as they work closely with each other and rely on each other's strengths.
I really like 'Azula and Katara: A New Path'. In this fanfic, it starts with a rather hostile encounter between them. But as the story progresses, they discover that they have more in common than they thought, especially when it comes to their family issues and the pressure they've faced in their lives.