One common theme is the clash of cultures. For example, when vikings cross over to a different world, they might have different values and beliefs compared to the native inhabitants, which leads to interesting conflicts.
One common theme is adventure. Vikings were known for their long voyages, so many viking fictions focus on their travels to new lands.
One common theme is adventure. Vikings were known for their travels, so stories often involve their voyages to new lands. Another is heroism. The Viking heroes like Ragnar are depicted as fearless in battle.
One common theme is adventure. Vikings were known for their travels and raids, and this is often central in viking historical fiction. Another is honor. Their code of honor within their clans and in battle is frequently explored.
A common theme is the struggle for acceptance. The minotaur, being a different and often feared creature, usually has to fight to be accepted in the new world or by the new group of characters it crosses over with.
One common theme is power struggles. Since there are different powerful characters from various planes, they often clash over control or resources. For example, in many fanfictions, two powerful planeswalkers might fight for dominance over a newly discovered plane.
Power is a common theme. Since Vegeta is extremely powerful, seeing how his power compares to characters from other universes is always interesting. Another theme is culture clash. For instance, when Vegeta enters the Harry Potter world, his Saiyan warrior culture clashes with the wizarding culture. Pride is also a theme. Vegeta's Saiyan pride often plays a role in how he interacts with characters from other fandoms.
Survival is also a big theme. Characters in the crossover fanfiction usually have to figure out how to survive in the presence of Annabelle, especially if they come from a world where they're not used to such malevolent supernatural forces.
One common theme is the idea of redemption. Raphael, who has a somewhat dark past in the original lore, often goes through a journey of redemption in these crossovers, especially with Simon's influence. Another theme is friendship. Despite their differences, they form a strong bond.
Another theme is power exploration. Naruto and Sakura have their own set of powers from the Naruto universe. When they are in a crossover, they often have to figure out how their powers work in relation to the new world they are in. This can lead to some really interesting battles and power - up moments as they learn to adapt and use their skills in new ways.
The theme of redemption also shows up sometimes. Despite being a fearsome character, in some crossovers, Deathwing might be given a chance to redeem himself. Maybe he caused a lot of destruction in his original world, but in the crossover world, he has the opportunity to do good deeds and change his reputation.