In this fanfiction situation, they could end up using their knowledge of the future to help people in the past. For example, Ash could use his Pokémon training skills to solve some problems that people in the past were having with wild Pokémon. Serena could perhaps use her cooking skills to introduce new recipes. And together, they would be constantly looking for clues or a magical item that could send them back to their time.
Perhaps in this fanfiction, the main antagonists are people from the past who are threatened by Ash and Serena's presence. They see them as a danger to their way of life or their power. For example, a local tyrant in the past might not like Ash's ideas about fairness and equality when it comes to Pokémon training. Serena's modern - style cooking might also be seen as a threat to local food traditions, so those who uphold these traditions could oppose them and become antagonists.
They might try to change some past events. For example, Naruto could prevent some of the early tragedies in the village. Hinata might also get the chance to be more assertive earlier in her life, since she has Naruto by her side this time. They could also have an impact on the relationships between different ninja clans.
It could be about someone who is 'stuck' during Christmas. Maybe they're stuck in a snowstorm and have an adventure trying to get home for Christmas. Or perhaps they're stuck in a small town and discover the true meaning of Christmas there.
One idea is that a child is stuck in a Christmas tree lot. He got lost while his family was picking out a tree. As he's stuck there, he befriends the lot owner and learns about the hard work that goes into getting Christmas trees ready for families. In the end, his family finds him and they have a great Christmas with a special tree.
Maybe it's an overzealous Hogwarts professor who suspects Harry of being a dark wizard due to his strange appearance in the past. This professor could constantly try to thwart Harry's attempts to find a way back to his time and also try to drive a wedge between him and Snape.
Well, one possibility is that Harry could accidentally trigger a time - turning spell while in Hogwarts. Once stuck in the past, he might have to deal with a younger Severus Snape. Maybe he tries to change some events related to Snape's past to improve their relationship in the future.
In the 'Ben and Mal's daughter stuck in the past fanfic', it's possible that she becomes a key figure in an important event in the past. She could use her knowledge from the future to help or hinder certain events. Maybe she gets involved in a battle that was crucial for the establishment of Auradon. Her presence could change the outcome in unexpected ways. She might also form new relationships with characters in the past that could have an impact on her family's future relationships.
Their relationship could develop more quickly. In the past, without all the distractions and responsibilities of their original time, they could focus more on each other. Hinata might be more confident in expressing her feelings earlier, and Naruto would likely be more receptive since he's away from his normal hectic life.
You could also do some research on the author. Sometimes knowing their writing style, their other works, or their general ideas can give you insights into the particular story you're having trouble with. For example, if the author often uses foreshadowing in a certain way, you might be able to figure out what's coming next in the story based on that knowledge.