Well, there's often the theme of friendship or rivalry. Crossover characters may form new friendships with Earthlings or become rivals with existing Earth characters. Another common theme is the clash between different technologies or magic systems. For example, a magical character from another world trying to understand Earth's advanced technology, or vice versa.
One common theme is culture shock. When characters from different worlds come to Earth, they experience new cultures, languages, and social norms. Another theme is power dynamics. If powerful characters from another realm arrive on Earth, it changes the balance of power. Also, the theme of adaptation is often present as the characters have to adapt to Earth's environment.
A common theme is the clash of cultures. When characters from different universes meet in Middle Earth, there's often a struggle to understand each other's values and ways of life.
One common theme is culture shock. RWBY characters are from a very different world, so when they come to Earth, they're amazed by things like the variety of foods and different forms of entertainment. Another theme is adaptation. They have to learn new skills to survive on Earth, like using modern technology.
One common theme is the clash of cultures. Since it's a crossover, different cultures from the FFXIV world and the other crossed - over world collide, which makes for interesting storylines.
One common theme is the clash of different powers. For example, when Venom crosses over with other superheroes or villains, there's always a comparison and sometimes a struggle between their abilities. Another theme is the exploration of new relationships. Like in crossovers with Spider - Man, it's about their complex relationship, whether they're enemies or allies at different points.
Often, there's a theme of friendship and cooperation. In crossovers like Boruto with the Attack on Titan world, Boruto and his friends have to work together with the characters from that world, like the Survey Corps. They learn from each other and grow stronger together. Also, a sense of discovery is common. Boruto and his team are often in a new world they don't understand, and they have to figure things out, which makes for an interesting read.
One common theme is the clash of different magic systems. For example, when Skyrim crosses over with a magical - based world like Harry Potter, you get to see how the Restoration magic in Skyrim compares to the spells in Hogwarts. Another theme is cultural differences. If Skyrim crosses with a world like Middle - earth, the Nordic culture clashes and sometimes blends with the cultures of elves and men in Middle - earth.
One common theme is the clash of different worlds. Since it's a crossover, you often see the unique settings of each original work collide. For example, if it's a Durarara!! and Bungo Stray Dogs crossover, the normal yet chaotic Ikebukuro and the world full of gifted individuals with special abilities from Bungo Stray Dogs create a lot of interesting situations.
Another theme is the exploration of different universes. In many crossovers, Supergirl might find herself in a different universe where the rules are different. This allows for interesting plot developments as she has to adapt to new situations and new versions of familiar characters. It also gives a chance to explore what makes her character unique compared to others in different universes.
One common theme is the clash of cultures. Since it's a crossover, different worlds or time periods collide, and the musketeers have to adapt.
One common theme is power - how the characters from 'the breaker' interact with characters from other universes in terms of their powers. Another could be friendship. When different characters cross over, new friendships can form.