One common theme is the power struggle. In both Naruto and Rosario Vampire, power is important. In the fanfiction, the main character often has to balance their power and use it to protect the members of the harem. For example, Naruto's ninja powers against the various supernatural threats in the Rosario Vampire world.
Love and loyalty within the harem is also a major theme. The characters in the harem have to develop feelings for the main character (often Naruto) and show loyalty. There are also rivalries within the harem, which add drama. For instance, characters from Naruto and Rosario Vampire might compete for Naruto's attention, and this creates a lot of emotional and relationship - based storylines.
Another theme is the exploration of different cultures. Naruto comes from a ninja - based culture, while Rosario Vampire has a supernatural - school - based culture. The fanfiction explores how these two cultures clash and blend, especially when it comes to the harem. It's interesting to see how the characters adapt to each other's ways of life.