Sam could face issues like not knowing the exact historical details and having to blend in without raising suspicion. Also, he might struggle with the technology and social norms of that time.
He might have trouble adapting to the past and avoiding changing events that could have unforeseen consequences.
One challenge Clark might face is changing the future too much. If he alters significant events, it could have unforeseen consequences. For example, if he saves someone who was supposed to die, that person's actions in the future might cause more harm than good. Another challenge is dealing with his younger self. He has to be careful not to expose his identity or give away too much information. Also, he might face challenges in terms of fitting into the time period. The technology and social norms of the past could be very different from what he's used to in his own time.
One challenge could be not being able to change things as easily as he thinks. Time might have its own rules. Another is that he might accidentally change something that has a much worse consequence later.
Lots of things could change! Maybe Sam alters events that were supposed to happen or meets people from the past in unexpected ways.
In fanfiction, if Naruto goes to Kiri, he would face challenges related to the village's internal politics. There could be different groups within Kiri with their own agendas, and Naruto's presence might disrupt the balance. He might also struggle with the language barrier or cultural differences. For example, some of Kiri's traditions might be completely foreign to him, and he could accidentally offend someone. And of course, there are always the powerful and dangerous missing - nin from Kiri that he could run into.
Harry could face the problem of balancing his knowledge from the future and not revealing too much. There could also be issues with dealing with younger versions of his friends and enemies. And, of course, he'd have to navigate the magical world of the second year without raising too many suspicions.
One challenge Naruto might face is getting used to magic. Since he's used to chakra - based powers, the concept and use of magic could be quite confusing at first. Also, the rules and culture of Fairy Tail might be different from what he's used to in Konoha. He'd need to adapt to the guild's way of doing things.
Well, in some fanfictions, they go back in time and accidentally interfere with the lives of their younger selves. This creates all sorts of chaos as they have to deal with the younger Sam and Dean's reactions to seeing their future selves. Or perhaps they go back to a crucial moment in the history of the hunter community and end up changing the rules and relationships within it. It could also be that they go back in time to a specific era like the Wild West and have to deal with supernatural threats that were prevalent during that time.
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.
He could meet his grandparents. Meeting them would give him a deeper understanding of his family history and the roots of the Winchester family's connection to the supernatural. Another interesting character would be young John Winchester. Sam could see what his father was like before all the tragedies happened and maybe even influence him in a positive way.
One challenge would be making friends. The students at Mahoutokoro might have different values and social norms compared to Hogwarts. He might find it difficult to fit in and find a group he belongs to. Additionally, the academic system could be very different. The grading system, the types of courses, and the way knowledge is imparted could all pose difficulties for Harry.