A prevalent theme is adaptation. Ironman has to adapt to the new rules and environments of the crossed - over universe. For instance, in a crossover with a historical - based world, he has to work within the limitations of that era's technology while still using his own skills. Also, identity exploration can be a theme. How does Ironman maintain his identity as a hero in a completely different world? This is often explored in these fanfics.
One common theme is the exploration of their age difference in a romantic context. Another could be how they support each other in different ways compared to the traditional superhero relationships. Also, the idea of mentorship turning into something more is often explored.
One common theme is the clash of their different personalities. Batman is dark and brooding, while Iron Man is more flamboyant. Their interactions often explore how they deal with this contrast.
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.