Yes, if the title 'aang is a firebender fanfic' is to be taken literally, then aang is likely a firebender within this fanfic. However, without actually reading the fanfic, there could be some sort of plot twist or alternative interpretation that might make it not so straightforward.
Since 'lemon' fanfic is inappropriate, we can't really discuss the characteristics in that context. But in general, aang is known for his kind - hearted and adventurous spirit. Toph is strong - willed and independent. In proper fanfic, these traits are often explored in their relationship, like how aang's optimism might contrast with toph's more cynical view at times.
He could meet his future self. It would be an interesting encounter as he could learn from his future experiences and wisdom. Or he might meet people who have studied his past adventures and see himself through their eyes.
One common theme is family building. Writers often explore how they raise their children and pass on their bending skills. Another theme is adventure. Since they are both adventurous at heart, fanfictions might show them going on new quests together. And of course, there's the theme of love. It's about how their love endures and grows through different situations.
In fanfiction, their married life could be filled with adventures. They might travel together to different parts of the world, using Aang's airbending skills and Katara's waterbending. They would also be involved in helping the people, just like they did before getting married. Their love would grow deeper as they face new challenges side by side.
Possibly they are random villagers who were caught in the crossfire of a conflict between different bending factions. Aang, being the Avatar and a symbol of peace, feels responsible for burying them and ensuring their spirits are at peace.
He might start by observing. Aang is a curious person, so he'd likely watch how people live, how they interact with technology. He may also try to find other benders to learn about any new developments in bending. And he'd probably use his kind heart to make friends easily in this new future environment.
In fanfiction, they might have a peaceful life. Aang could focus on rebuilding the Air Nomad culture. Katara would be by his side, using her waterbending skills to help the new communities. They could also start a family, passing on their bending knowledge to the next generation.
Sokka could be one of the main characters. He might be the one helping Aang with his schoolwork, especially in subjects like history or math, using his own knowledge and quick thinking.