Cooperation is a frequent theme as well. Since crossovers usually involve different groups coming together, they have to learn to cooperate. In fanfictions where the X - Men cross with other powerful entities, they need to find common ground and work together despite their differences.
Power exploration is a big theme. Fem Harry has her own magical powers, and in the X - Men world, she has to figure out how they interact with the mutant powers. Also, acceptance is a theme. The X - Men are all about acceptance of differences, and Fem Harry has to find her place among them, whether it's being accepted as a non - mutant but still powerful being or finding a way to contribute to their cause. There's often a theme of fighting against prejudice too, as both she and the mutants face different forms of it in their respective worlds.
Power struggles are a common theme. The pillar men are very powerful, and when they cross over, there's often a fight for dominance. Another theme is adaptation. They have to adapt to the new world or new characters they encounter. For example, in some fanfics, they struggle to understand modern technology.
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.
One common theme is power exploration. Writers often explore how Tsuna's powers interact with the powers in the crossover universe. For example, in a crossover with a superhero universe, how Tsuna's unique abilities compare to superpowers like flight or super strength.