A major theme is adaptation. Harry has to adapt to a whole new set of rules and abilities as a senshi. This includes learning about the senshi world, its hierarchy, and how his magic as a wizard interacts with his senshi powers. There's also the theme of self - discovery as he figures out how this new part of himself fits into his overall being and his place in the world.
It depends on the plot of the fanfiction, but Hinata could also be a main character in a Naruto Outer Senshi story. Her shy but strong - willed nature could be developed further when she meets the Outer Senshi. Maybe she gets some special powers from them or has to help them understand the Naruto world better. And of course, Sailor Neptune, with her elegance and powers, would be a central figure in the story as well. Their relationship could be one of mentorship or friendship, which would add depth to the story.
One common theme is the crossover of powers. For example, Naruto's ninja techniques might be combined with the Sailor Senshi's magical powers. It creates a new and exciting dynamic.
I'm not entirely sure as I haven't experienced this visual novel specifically. But generally, a visual novel often has a story line involving characters. Maybe it's about female warriors with some unique characteristics, like in many visual novels where there are adventures, relationships, and challenges that the characters face.