Well, a common theme is often the adjustment. Harry has to adjust to Rias's world and vice versa. This includes getting used to different social structures, magic systems, and ways of life. Love against all odds is also a theme. Since they are from different universes, there are likely to be many obstacles in their relationship, but their love overcomes them. And of course, there's the theme of adventure. They might go on adventures together, using their combined powers and knowledge to face various challenges.
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.
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.
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 common theme is the 'what - if' scenario. Like what if Harry had realized his feelings for Hermione earlier in the series. Another theme is dealing with the challenges of their friendship turning into a romantic relationship while still being part of the wizarding world's battles.
The theme of redemption can also be found in some of these fanfictions. A character who has made mistakes in the past might use a love potion to try and make amends with someone they've wronged. But then they realize that using the potion is wrong in itself and they have to find a more genuine way to earn forgiveness. It shows their journey towards becoming a better person. Also, the power struggle between good and evil can be a theme when the love potion is used as a weapon by the bad guys. The good characters have to fight against the false love created by the potion to save their relationships and the wizarding world.
One common theme is the idea of forbidden love. Since Tonks is older and has different relationships in the original series, their love in fanfiction often faces obstacles from society within the wizarding world. Another theme is self - discovery. As they explore their feelings for each other, both Harry and Tonks also learn more about themselves.
One common theme is the struggle against the odds. Since their relationship is so unorthodox in the context of the original story, the characters often have to fight against the expectations and norms of the wizarding world. Another theme could be self - discovery. Harry might discover new aspects of his own personality through his feelings for Snape, and vice versa.
One common theme is the idea of redemption. Bellatrix, being a Death Eater, might be seen as redeemable through her love for Harry. Another theme could be the power struggle. Harry has his own power as the 'Chosen One' and Bellatrix has her loyalty to Voldemort, so the fanfiction might explore how their relationship affects this power balance. Also, there's often the theme of forbidden love, as their relationship would be considered highly inappropriate in the context of the wizarding world.
Well, the theme of self - discovery through love is quite common. In these fanfictions from the 2000s, characters might find out more about themselves while falling in love. And loyalty in love is another one. Characters stay loyal to their loved ones even in the face of danger. The concept of love at first sight also pops up a lot, where two characters meet and are immediately smitten with each other.