Romance can also be a theme. In some issei crossover fanfictions, he might develop relationships with characters from the crossed - over world. For instance, in a crossover with a magical girl anime world, he could end up having a romantic interest in one of the magical girl characters, which then adds an extra layer of story - telling as he tries to balance his feelings with his mission or place in the new world.
Conflict is a major theme. When Issei crosses over into another universe, there are bound to be conflicts, whether it's against villains from that universe or differences in ideology with the characters there. For instance, in a crossover with 'Attack on Titan', Issei might have to deal with the Titans and also the complex political and moral issues within the Walls. And of course, there's always the theme of survival as he faces new and dangerous situations in the crossover.
Many fanfictions focus on Issei's role within the devil society. How he rises in rank, earns respect, and changes the way things are done. There are also themes related to loyalty and friendship. His relationships with his friends and comrades are often explored, showing how they support each other in the face of various challenges in the dangerous world of 'High School DxD'.
Well, the theme of sacrifice is quite prevalent. Sometimes either Issei or Rias has to sacrifice something for the other or for the greater good. Another common theme is identity. Issei, as a human - turned - supernatural being, and Rias, as a devil, both struggle with their identities at times. Their relationship helps them to better understand themselves. And the theme of destiny often comes up, as if they are fated to be together and face all these challenges as a couple.
A significant theme is also self - discovery. Being reborn gives Issei a chance to look at himself and his role in the world differently. He might question his previous actions and strive to be a better version of himself. Additionally, alternate universe scenarios are common. The fanfictions may take Issei to different settings or situations that deviate from the original plot, like a world where the power balance is completely different or where certain events didn't occur as in the original story.
One common theme is friendship development. They often start as acquaintances or even rivals and gradually become good friends. For example, in many fanfictions, they work together on a project and through that, their bond deepens.
One common theme is the growth of their relationship. It often starts from a somewhat distant or one - sided view and develops into a deep and meaningful connection. Another theme is their combined strength in battles. Writers like to show how they complement each other in fighting against enemies.
One common theme could be emotional pain. Issei might be going through heartbreak, betrayal, or loss in the fanfiction. Another theme could be physical harm, perhaps from a battle or an accident.
A common theme is power. How Issei gains power from Amaterasu or how they combine their powers. For example, in many fanfictions, Issei's combat abilities are enhanced by Amaterasu's divine energy.
One common theme is power - up. Issei often gains new powers or abilities in these fanfictions. Another theme is romance, usually involving his relationships with different female characters in the series.
Power and responsibility are common themes. Since Issei is the son of Sparda, he often has great power, and he has to learn how to use it responsibly. Another theme is identity. He has to figure out who he is in relation to his father and his own place in the world. Family legacy also plays a big role as he is constantly living up to or trying to break free from the expectations of being the son of Sparda.