First, give your OC CPU a unique power or ability. For example, the power to communicate with machines. Second, define their personality traits clearly. Are they brave, shy, or mischievous? Third, think about their backstory. Maybe they were created in a special experiment. This will make your character more interesting.
One common theme could be friendship. Since the Neptunia series has a lot of characters, fanfics might explore the relationships between different CPUs. Another theme could be adventure, as they often go on quests in the game. Maybe in the fanfic, they have an adventure during the night.
One idea could be an OC CPU who has the power to manipulate time. She could use this power to try and prevent a major war in the Hyperdimension Neptunia world. Another could be an OC CPU that is half - human, half - goddess and has to struggle with fitting into both worlds and finding her true identity among the other CPUs.
I'm not sure of a specific one to recommend off - hand. You could try searching on fanfic websites like Archive of Our Own or FanFiction.net. There are likely many fanfics with that title or related to the concept, and you can read the summaries and reviews to find one that suits your taste.
One common trope is the 'fish - out - of - water' situation. The male SI is suddenly in the Neptunia world which is so different from his own. Another is the 'power - discovery' trope. He might find out he has a special power or ability that he didn't know about before. For example, he could have the power to control digital entities.
To write an engaging male self - insert (SI) in Neptunia fanfiction, first, understand the Neptunia universe. Know the different characters, their personalities, and the overall world setting. Then, give your male SI a unique personality. He could be brave yet with a sense of humor. Also, create a backstory for him. Maybe he's from another dimension or has a special ability that ties into the Neptunia lore. For example, if the Neptunia world has a lot of digital battles, your SI could have a unique hacking skill.
Common themes include the exploration of the Neptunia world from the perspective of an outsider (the gamer). This often leads to misunderstandings and comical situations at first. Also, the harem aspect brings in themes of love, friendship, and loyalty among the characters in the harem. And of course, there's usually a theme of the gamer character trying to find their place in this new and strange world while also dealing with the affections of the Neptunia girls.
In many 'Si Gamer in Neptunia Universe fanfiction', a major theme is adaptation. The gamer has to adapt to the rules, cultures, and technology of the Neptunia world. This can lead to some interesting situations where they have to learn new ways of combat or communication. Another theme that often pops up is the search for a way back home. The gamer may be trapped in the Neptunia universe and is constantly seeking a way to return to their original world, all while making an impact on the Neptunia universe itself.
There are several good ones out there. For example, some fanfictions explore how the gamer character uses their gaming knowledge to navigate the Neptunia world. They might introduce new game - like elements to the story. The harem aspect usually involves the main character forming relationships with various Neptunia girls, like Neptune, Blanc, and others. These relationships can range from friendly to romantic, and the fanfictions often play with different character dynamics.