Trust would be a major challenge. The team has seen her as an enemy for so long. Also, she'd have to adjust to their way of doing things which is very different from how she was trained in the Fire Nation.
One of the main challenges for Azula joining Team Avatar in fanfiction is her own pride. She's used to being in control and being at the top. In the team, she'd have to be more of an equal. Also, she might struggle with the idea of working towards goals that are not about Fire Nation domination. Her firebending skills, while useful, might also be a source of tension as the team might be wary of her power. She would need to prove that she has truly changed and wants to be part of their group for positive reasons.
Azula would face challenges like adapting to the team's values. Team Avatar values things like friendship and equality, which are not really emphasized in the Fire Nation. And she'd have to deal with the fact that she has hurt some of the team members in the past, so getting them to accept her would be tough.
She could bring a whole new dynamic. Her powerful firebending skills would be a great asset. But also, her complex personality might cause some internal conflicts within the team as they'd have to learn to trust her, given her past actions.
He would face a huge ideological challenge. Madara has very extreme views about power and the ninja world order, which are completely different from Naruto's beliefs in friendship and cooperation. Naruto would constantly have to struggle not to be swayed by Madara's ideas.
One challenge could be adjusting to the different technology and ways of the past. For example, if they go back to a time before modern ninja tools, they'd have to rely on more basic equipment. Another challenge would be not accidentally changing the future in a bad way. They might also face challenges like being suspected as spies since they are from the future.
One challenge could be adapting to the team's dynamics. Team 8 has its own way of operating, and Naruto is used to his own style with his friends. For instance, they might be more methodical in their planning, while Naruto is more of a 'jump - in - and - figure - it - out' type. So, he'd have to learn to follow their lead sometimes.
Naruto would face quite a few challenges. For one, his growth might be stunted as the team overprotects him. He could also face ridicule from outside the team who might see him as a weakling now. Moreover, he has to deal with the internal team dynamics where Sasuke may not be fully accepting of his new role and Naruto would have to prove himself constantly.
One challenge could be dealing with jealous teammates. Since Percy is so talented in the water, some might envy his skills. Another is the pressure to always win. Everyone expects the team to do great with Percy as captain. Also, he might face difficulties in getting the team to work together if there are different personalities.
In the ANBU after leaving Team 7, Naruto will be confronted with numerous challenges. Physically, the missions are likely to be more intense, requiring him to further develop his skills and combat abilities. He'll need to learn new stealth techniques and master different types of weapons. Emotionally, he'll face isolation. ANBU work is solitary, and Naruto, who thrived on the camaraderie of Team 7, will have to adjust to being alone most of the time. Additionally, he'll be faced with complex political situations within the ANBU organization. He'll have to navigate through power struggles and different factions, all while trying to stay true to himself and his values.
One idea could be a story where the two teams compete in a friendly but intense tournament. They might face challenges that test their bending skills in new ways.
A lesser - known but really interesting fanfic is 'Azula in the Spirit World'. Here, Azula gets lost in the spirit world and has to find her way back. During her journey, she encounters different spirits that force her to confront her own spirit and the darkness within her. It's a very unique take on her character and offers a lot of depth to her already complex personality.
One great 'Avatar Azula fanfic' is 'The Redemption of Azula'. It shows a different side of Azula, exploring her possible path to redemption. It has really interesting character development and a great plot that keeps you hooked.