Since I don't really promote or encourage 'lemon' fanfiction which often has adult - themed content, I can't accurately say what the common themes are in that specific type. But in general Harry and Rias crossovers might have themes like power dynamics, as Harry has his own set of magical abilities from the Harry Potter world and Rias has her devil powers in High School DxD. Another could be cultural clash, as they come from very different fictional settings.
As 'lemon' fanfiction is not something I can fully engage with due to its often adult nature, we can focus on more general themes that could be in a non - 'lemon' Harry and Rias fanfiction. One theme could be the exploration of new magical or supernatural elements. Harry could introduce some of his unique spells and magic concepts to Rias' world, and vice versa. Also, there could be a theme of teamwork, where they have to work together to face a common threat that crosses over from both of their fictional universes.
I should note that 'lemon' fanfiction involves adult - like scenarios which are not appropriate to discuss in detail. However, for non - 'lemon' aspects in Harry and Rias fanfiction, themes could include friendship development. Imagine Harry, who is used to a very different social structure in Hogwarts, trying to fit into the world where Rias is from. There could also be themes of mutual respect and learning from each other's strengths and weaknesses.
A common theme is the clash of different cultures. Harry comes from the wizarding world with its own set of rules and traditions, while Rias is from a world of devils. Another theme is power exploration. Harry has his magic, and Rias has her devil powers, and in fanfictions, they often explore and combine their powers in new ways.
A recurring theme is the discovery of new things. Harry discovers new aspects of Rias' world and vice versa. It's like exploring uncharted territory. Also, there's often a theme of loyalty. As they face difficulties, they show loyalty to each other which is an important aspect of their relationship in these fanfictions. These themes together create a rich and interesting reading experience.
One common theme could be the clash of different cultures. Harry comes from the wizarding world in England with its own set of rules and traditions, while Rias is from a different kind of supernatural world. Another theme might be the power dynamics. Both characters have their own forms of power, and exploring how they interact with each other in terms of power and support in their relationship could be interesting. Also, there could be a theme of discovery, as they learn about each other's worlds and abilities.
Friendship is also likely to be a theme. They share a special bond, and the fanfiction might delve into how they communicate, confide in each other, and have fun together. For example, they could have girl - talk sessions about their crushes or their future plans within the devil society.
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.
Another theme could be their relationship within their group or the politics of the supernatural world they inhabit. How they interact with other members, make alliances, and face off against enemies can make for great fanfiction. It's a more complex and engaging way to write about them compared to inappropriate themes.
One common theme is the exploration of their deep, passionate love. Since they have a strong connection in the original series, fanfiction often delves deeper into the romantic and physical aspects of that connection.
A prevalent theme is the exploration of power. Harry and Edward both have unique abilities, and in these fanfictions, they often have to figure out how to use or control their powers. Also, the theme of loyalty shows up a lot. They have to decide who they can trust and be loyal to in different situations. This creates a lot of interesting plotlines and character development. For example, in some stories, they might be tested by a new character who seems friendly at first but has ulterior motives.
Often, the theme of heritage comes up. Harry's connection to Lily and how that relates to certain events, and the lemon might be used as a symbol related to that heritage. Also, friendship or loyalty can be themes, as they all interact in different ways within the fanfiction.
Love and friendship are common themes. Writers often explore how Harry and Penelope's relationship develops, whether it's a budding romance or a strong platonic bond.