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.
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.
There are probably some 'Harry Potter and Rias Love' fanfictions. Usually, in these types of crossovers, the authors might start by establishing how the two characters from different universes meet. Maybe Harry gets transported to Rias's world or vice versa. Then, they would build on their relationship, perhaps starting as friends and gradually developing into something more. It's all about the creativity of the fanfiction writers, and different writers will have different takes on this unique pairing.
One idea could be that Harry Potter, while on a mysterious magical journey, stumbles upon the world where Rias Gremory exists. They initially misunderstand each other due to their different magical backgrounds. Harry is used to wands and spells, while Rias has her own demonic powers. But as they get to know each other, they start to learn from one another and combine their powers to face a common enemy that threatens both their worlds.
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.
His friends from Hogwarts could still be his allies. Hermione would use her intelligence to help him understand his new situation. Ron would be there for moral support. Additionally, any friends or allies that Rias has in her world could also become Harry's allies. They might be magical creatures or other powerful mages.
Since Harry has dealt with complex magical artifacts like the Marauder's Map, he could help Rias in her search for powerful items in the Highschool DxD world. His expertise in this area could lead to Rias obtaining artifacts that significantly enhance her development. For example, she might find an item that allows her to communicate with her allies more effectively or one that gives her better control over her devil powers. Harry's magic would not only add new skills to Rias but also open up new avenues for her growth and exploration.
One possible fanfiction idea could be that Rias' sick brother is a key factor in a power struggle within their supernatural world. Naruto, being an outsider with his own set of skills and values, could get involved. Maybe he forms an alliance with Rias to protect her brother from those who would use his illness to their advantage.
In such a fanfiction, Harry Potter might grow up with a very different set of values. Rias is from a different fictional universe with her own code of conduct. He could be trained in devil - related magic or combat skills that are unique to her world. Maybe he would be more assertive in relationships as Rias is quite bold in that aspect.
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.