Power is a common theme. Xehanort is often a power - hungry character, and in crossovers, his pursuit of power is pitted against the powers and rules of the new universe he enters. For example, if he crosses over into a universe with a strict moral code regarding magic use, his power - hungry ways will be challenged.
Redemption can also be a theme. Some fanfictions explore the possibility of Xehanort finding redemption in a new world. Maybe in a world full of kindness and forgiveness, he starts to question his dark past and actions. This often involves complex character development as he struggles between his old self and the new influence of the crossover world.
Conflict is always there. It could be a conflict between Xehanort and the heroes of the crossover world. Or it could be an internal conflict within Xehanort himself as he tries to adapt to the new world while still holding onto his original goals. This conflict drives the plot forward and makes for an interesting read.
There's also 'Xehanort's Journey through Kingdom Hearts Crossovers'. This fanfiction takes Xehanort on adventures across different Kingdom Hearts - like worlds that have been cross - overed with other game universes. It's interesting to see how Xehanort's dark plans are affected by the new environments and characters he encounters.
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.
Power struggles are a common theme. Since Frieza is extremely powerful, when he's crossed over into other universes, there's often a battle for dominance. For example, in crossovers with superhero universes, he'll fight against heroes who also have great power.