One common theme is the power struggle. Since both Naruto and DxD have complex power systems, in the fanfiction, Naruto often has to prove himself in the DxD world which has its own set of powerful beings. Another theme is friendship. Naruto is known for his ability to make friends, and in these crossovers, he forms new friendships in the DxD world.
One popular plotline could be Naruto being transferred to the Highschool DxD world and his ninja skills making him stand out among the devils and angels. This leads to various characters being attracted to him, forming the harem. Another could be Naruto having to use his chakra to solve the power struggles in the DxD world while also dealing with his newfound romantic interests.
Another good one could be 'Naruto and the DxD Harem Adventure'. It starts with Naruto finding himself in a strange situation in the DxD world. As the story progresses, he starts to gain allies and also the affection of multiple female characters. The fanfiction does a good job of blending the different mythologies and power systems from both Naruto and DxD.
One thing that makes this type of fanfiction appealing is the contrast. Naruto, a character known for his loyalty, is betrayed in his own world. Then he's thrown into the DxD highschool where there's a whole new set of rules and relationships. The harem part means there are multiple characters with different personalities that can have various interactions with Naruto. It's also about seeing Naruto grow and change in this new environment, perhaps learning to trust again through his relationships with the harem members.
Power - building is a common theme. The OC usually starts off with some sort of special ability or knowledge and uses it to gain more power in the Highschool DxD universe. This power - building often attracts the attention of various female characters, leading to the formation of the harem.
The OC can stand out by having a distinct personality. For example, if all the other harem members are bold and outgoing, the OC could be more reserved and intellectual. This contrast would make them interesting. Also, a unique skill or ability, like being able to communicate with spirits, would set them apart.
One idea could be the OC having a unique power that attracts the various characters in the harem. For example, if the OC can manipulate time in a certain way, it could lead to some really interesting situations. Maybe they use this power to save a member of the harem from a dangerous situation, which then makes that character develop strong feelings for the OC. Another plot idea is the OC being from a different dimension and having to adapt to the Highschool DxD world while building relationships with the harem members.
Naruto and Ravel might start off as strangers. Naruto would be the curious outsider, and Ravel would be intrigued by his different appearance and powers. Then, Naruto could interact with other DxD characters. For example, he might have some friendly competitions with Issei. As the harem forms, Naruto would have to balance his relationships with different girls. He might have deep conversations with Ravel about their different worlds and values.
The story could unfold in an interesting way. Naruto's ninja skills would be quite a novelty in the Highschool DxD world. Ravel, being a devil, would initially be curious about Naruto. The harem aspect would come into play as Naruto's kind heart and heroic deeds draw in more female characters. They would have to navigate the complex social and power structures of the DxD world. Naruto might teach them some of his ninja philosophies, and together they would create a unique dynamic within the group, facing challenges both from the devils' internal politics and external threats.
There's also 'Rising of the OC in DxD'. In this fanfiction, the OC is a reincarnated being. He uses his past - life knowledge to his advantage in the world of Highschool DxD. His harem includes characters from different factions, and the story delves into how he manages to keep them all together while facing various challenges from the supernatural world.