One of the main themes is friendship. In many heero yuy crossover fanfictions, Heero forms new friendships with characters from other universes. This helps him in his new adventures. Also, the theme of conflict is quite common. Heero often finds himself in the middle of battles or disputes in these crossovers, either because of his own beliefs or because he's trying to protect someone. And then there's the theme of exploration. Heero gets to explore new worlds, technologies, and magic systems which makes for really exciting stories.
In heero yuy crossover fanfiction, the theme of identity crisis can be seen frequently. Since he is in new and strange crossovers, Heero may question his purpose and values. Another theme is the power struggle. When combined with other universes, there are often different power dynamics at play, and Heero has to navigate through them. Moreover, the theme of redemption can also be present. Heero may try to make up for past mistakes or the mistakes of the characters he meets in the crossovers, which adds depth to the stories.
One great heero yuy crossover fanfiction is 'Heero's New World'. It combines the character of Heero Yuy with a different anime universe and creates an interesting plot where Heero has to adapt to new rules and make new alliances. Another is 'Heero and the Magic Realm' which throws Heero into a fantasy setting full of magic and mystery.
It could be that Harry Potter somehow gains the skills and personality traits of Heero Yuy. Maybe he becomes a more stoic and calculating wizard, using Heero's combat strategies in magical battles.
There could be. However, I'm not entirely sure which specific platforms would have it. Some other smaller, niche fanfiction communities might also host such works. You could also try reaching out to Harry Potter or Gundam Wing (where Heero Yuy is from) fan groups on social media and ask if they know of any.
One common theme is the exploration of their deep emotional bond. Since they are often paired in these fanfictions, the writers like to dig into how they feel about each other on a very profound level. Another theme is the idea of overcoming obstacles. Whether it's their own personal issues or external factors, they usually have to fight through something to be together.
One common theme is trust. Since they are comrades in the Gundam Wing universe, many fanfictions explore how they build trust in each other. For example, in some stories, they start off as rivals but gradually come to trust one another completely.
A really creative plot is in a fanfic where the afterlife concepts from Yuy Yu Hakusho are merged with the demon hierarchy in Blue Exorcist. The story has Yusuke being sent on a mission by Koenma to help the Blue Exorcist crew deal with a demon that is trying to disrupt the balance between the human world and the demon world in a completely new way. It plays with the idea of different levels of existence and power in both universes.
There's also 'Cross - Realm Defenders'. In this fanfiction, the demon - slaying aspects of both series are the focus. Characters from both shows form a sort of alliance. It shows how Kuwabara from Yuy Yu Hakusho might react to the more formal exorcist system in Blue Exorcist, and how the Blue Exorcist characters would view Kuwabara's unorthodox fighting style. It's full of action and some really cool battle scenes.
One common theme is adventure. Since the Valerian universe is full of exciting and unknown elements, when crossed with other universes, it often leads to epic adventures. For example, a crossover with a space - opera like Star Trek could result in a story where Valerian and his companions explore new galaxies together with the Star Trek crew.
One common theme is the clash of different powers. For example, when the Incredibles cross with other superhero groups, there's always the exploration of how their powers compare and contrast.
Power struggle is a common theme. Starscream is often power - hungry in the original Transformers series, and in crossovers, this trait is magnified when he enters new universes with different power hierarchies. For example, in a crossover with the Lord of the Rings, he might try to claim the One Ring to gain ultimate power.