One big challenge could be adapting to the toad's way of life. Their environment and culture are very different from what Naruto is used to in Konoha. For example, he may have to deal with different kinds of food and living conditions while training. Also, the training methods of the toads are likely to be very intense. They might push Naruto to his limits in order to unlock his hidden potential.
In the fanfic, Naruto may face the challenge of competing with other students or trainees of the toads. There could be some rivalry among them, and Naruto has to prove himself not only to the toads but also to his peers. Moreover, the toads may have high expectations for Naruto, especially considering his status as the Jinchuriki of the Nine - Tails. This pressure can be a huge obstacle for him to overcome during the training.
He might face challenges like mastering new jutsu. The toads have unique techniques that Naruto has to learn, which could be physically and mentally exhausting.
One challenge could be readjusting to the village life. After intense training in a different environment, the daily routines and social norms of Konoha might seem strange. Also, he may face jealousy from some of his peers who haven't advanced as much in their skills during his absence. Another challenge could be dealing with the new threats that Konoha might be facing, and having to prove himself all over again to the village elders that his new skills are useful for the village's protection.
One challenge could be facing powerful new enemies. In fanfiction, authors often create unique and formidable foes for Naruto. These enemies might have strange powers or abilities that Naruto has never encountered before, forcing him to quickly adapt his fighting style. Another challenge could be the moral dilemmas. He might have to make tough decisions in the chaos of war, like whether to sacrifice a few to save many.
Well, in some fanfictions, Naruto leaving Konoha for training means he has to face new enemies outside the village. These enemies might be more powerful or have different fighting styles that he's not used to. Also, he has to manage his resources on his own. Without the support of Konoha, getting food, water, and training equipment becomes a problem.
One challenge could be the language difference. Even though English is spoken in both places, the accents and some local slang in England might be confusing for Percy. For example, he might struggle to understand what people mean when they use terms like 'bloke' or 'cheeky'.
One challenge could be the lack of the traditional focus for his magic. Without a wand, he might struggle to direct his magic precisely. For example, when he tries to cast a simple levitation spell, he may end up making objects fly out of control.
One challenge could be the physical exhaustion. Jiraiya's training is likely to be very demanding physically, and Naruto might struggle to keep up with the intense workouts.
One of the main challenges could be the lack of immediate feedback. When training alone, Naruto doesn't have a partner to correct his mistakes right away. For example, if he's working on a new jutsu, he might not realize he's performing it slightly wrong until it fails completely. Also, he has to motivate himself without the competition or support of his peers. He might get bored or frustrated easily since there's no one around to cheer him on or push him further.
The legal system in some fanfic settings might not recognize slavery as an illegal practice immediately. Anakin would have to fight through bureaucracy and convince lawmakers to change the laws. There could also be cultural acceptance of slavery in certain areas, and Anakin would have to overcome the deeply - rooted beliefs of the people there.
Chakra depletion is also a big issue. With all the chakra - control exercises and sparring, Naruto could run out of chakra quickly. This would not only limit his ability to practice jutsu but also leave him vulnerable in case of an unexpected attack during training.
One challenge could be the difference in their natures. Madara is an Uchiha with a more cold and calculating style, while Naruto is more impulsive and emotional. Adjusting to Madara's training methods which are likely to be strict and demanding could be tough for Naruto.