Well, they could find out that there's a hidden world within the Polar Express itself. A world where all the lost toys go. Hero Boy and Hero Girl decide to help the toys find their way back to the children they belong to. This would involve a lot of exploration inside the train, finding secret passages and solving puzzles. Or perhaps they discover a time - traveling aspect of the Polar Express and end up in different Christmas eras, seeing how the celebration has changed over time and bringing back some old - fashioned Christmas spirit to the present day.
A plot could be that Hero Boy and Hero Girl are the only ones who can hear a strange melody that's leading the Polar Express astray. They follow the sound to find out that it's an enchanted instrument that's been stolen from the North Pole's music hall. They have to retrieve it before Christmas is ruined. Along the way, they face challenges like crossing a frozen lake filled with magical creatures that try to stop them. Their friendship and courage would be put to the test as they strive to complete their mission.
One plot idea could be that Hero Boy and Hero Girl go on a new adventure in the Polar Express to a magical land that's not the North Pole. They could encounter new characters, like a talking snowman who guides them through a forest of ice sculptures. Another idea is that they have to save the Polar Express from a dark magic that's making it lose its magic power. They would have to work together, using their unique qualities. For example, Hero Boy's determination and Hero Girl's kindness to break the spell.
One way their relationship could progress is if they start to understand each other's fears better. Say Hero Boy is afraid of heights and Hero Girl helps him overcome that when they have to climb a tall ice tower on an adventure. This act of kindness and support would deepen their connection. Also, if they get separated from the Polar Express and have to rely on each other to find their way back, it would strengthen their bond. They might learn more about each other's families and dreams during this time, which would bring them even closer.
Perhaps she forms an unlikely alliance with a villain-turned -hero. They could team up to take on a greater evil in the 'My Hero' universe. Their different perspectives and abilities would make for an interesting story arc.
One plot idea could be a villain who is secretly a double agent, working for the heroes all along. They infiltrate the villain group and gather information. Another idea is a hero who loses their quirk and has to rely on their wits and combat skills alone, like Batman in the DC universe. And a villain - hero team - up to face a greater threat that neither can handle alone.
An interesting plot could involve an anti - hero who has a hidden agenda that is not entirely evil but also not in line with the traditional hero goals. Maybe they are trying to expose a corrupt part of the hero society from within. They could form alliances with some villains who also have a vendetta against the corrupt elements. This would create a complex web of relationships and moral dilemmas.
How about a fanfiction where the villains and heroes have to team up? There could be a greater threat, like an alien invasion or a powerful quirk - user who is out to destroy the entire world. The heroes and villains realize that they can't defeat this threat alone, so they put aside their differences. This would lead to some really interesting character interactions, like Bakugo having to work with Shigaraki and both of them learning to respect each other's skills despite their opposing natures.
One plot idea could be that the female characters in My Hero Academia gain magical girl powers in addition to their quirks. They then have to balance using both powers while facing a new, more powerful villain. For example, Ochaco could use her zero - gravity quirk along with her new magical girl power of creating energy shields.
One plot idea could be Naruto being betrayed by the Hidden Leaf Village. He then decides to use his powers for his own gain rather than for the village. He might form his own group of outcasts and fight against the village's corrupt leadership.
Maybe a plot where a group of Warhammer heroes are sent to the Hero Academia world to stop a chaos invasion that is starting to seep into that realm. They have to work with the local heroes, sharing their knowledge of dark magic and ancient weapons to combat the new threat. This would involve a lot of cultural clashes and learning experiences between the two groups.
One idea could be Naruto suddenly finding himself in a world similar to 'My Hero Academia' and having to adapt to their different power systems. He might start teaching his ninja skills to the students there, creating a whole new dynamic.
One idea could be a time - travel plot where a character from the future goes back to the Naruto Hero Z era to change a major event. This could lead to all sorts of new alliances and rivalries.