Well, for a Sharingan Naruto Rosario Vampire fanfiction, a possible plot could be that Naruto is transported to the Rosario Vampire universe and his Sharingan gives him an edge in understanding the complex vampire and monster society there. He could form alliances with some of the characters like Tsukune. The beauty of fanfiction is that it can take many different directions, so different authors might explore different aspects of how Naruto's abilities and character traits interact with the Rosario Vampire setting.
In a Sharingan Naruto Rosario Vampire fanfiction, the author might play around with the idea of Naruto having to hide his Sharingan abilities in the Rosario Vampire world at first. As he gets more involved, he could use it to his advantage, for example, in battles against other powerful creatures. There are also opportunities for character development, like Naruto learning about the different values and cultures in the Rosario Vampire world compared to his own ninja world, which could lead to some really interesting storylines.