The balance can also be achieved by making the OC need to train and develop their Rinnegan powers gradually. They start with basic abilities and as the story progresses, they learn more complex and powerful ones. This way, they face challenges appropriate to their current power level at each stage of the story, and the power growth feels more natural and not overly sudden.
One idea could be an OC with the Rinnegan being a transfer student to the Highschool DxD world. Their Rinnegan abilities could attract the attention of different factions. For example, they might be able to use the Preta Path to absorb the demonic energy attacks of the devils, which would both intrigue and alarm the devils in the school.
A story could revolve around a Naruto character with the Rinnegan who is mistaken for a new type of supernatural being in the Highschool DxD world. As they navigate this new environment, they might have to hide their true identity at first. But as the story progresses, they could reveal their powers and form alliances with some of the factions in DxD. They might also introduce some ninja concepts like chakra and jutsu to the DxD characters, which could change the power dynamics in that world.
Well, if Naruto had the Rinnegan in the Highschool DxD world, the power dynamics would change drastically. Naruto's abilities with the Rinnegan could make him a dominant force in the battles against the devils and other supernatural beings. He might introduce new jutsu styles that could shock everyone in the DxD universe. Maybe he'd use his Rinnegan to quickly analyze the powers of his opponents and come up with strategies on the fly.
They could be a new student at the high school. Their Rinnegan powers would make them stand out immediately. Maybe they would be scouted by one of the factions like the devils or the angels because of their potential power.
The Rinnegan OC can stand out by having a moral code that is different from the typical characters in the gamer fanfiction. For instance, they might use their powers not just for victory but also to protect the virtual world's inhabitants in a more selfless way, which would make them a more complex and interesting character.
As a transfer student, Naruto with the Rinnegan would change the balance of power in the school. He could be a target for some groups who want to study or steal his power. At the same time, he could be a great asset to the school in protecting it from external threats. His Rinnegan - which gives him abilities like seeing the chakra flow of others - would be very useful in detecting hidden enemies or spies within the school. This would create a lot of drama and action in the fanfiction.
In fanfiction, one way to combine EMS (Eternal Mangekyou Sharingan) and Rinnegan powers could be through a powerful jutsu. For example, a character might create a new form of ninjutsu that draws on the unique abilities of both. The EMS could provide enhanced visual perception and the ability to cast powerful genjutsu, while the Rinnegan could offer control over different elements and the six paths techniques. They could be combined in a way that allows for teleportation between different dimensions, using the space - time manipulation aspects of both powers.
Give your OC a unique backstory. For example, they could have obtained the Rinnegan through a strange accident or a family curse. This will make their character more interesting.
The OC, as the Son of Sparda, could fit in by being a new and powerful entity. He might be initially seen as an outsider, but his powers would quickly draw attention. He could become involved in the power struggles between the different factions in Highschool DxD. For example, the devils might try to recruit him, while the angels might be wary of his presence. His unique abilities would also make him a valuable ally or a fearsome opponent. And in the high school setting, he could be a bit of an enigma to the other students, both human and supernatural.
An OC could stand out by having a special skill or talent. Maybe the OC is an amazing cook and can make the most delicious food. In a world where battles and magic are common, this simple yet appealing talent could draw in characters from the series. They would be intrigued by this different side of the OC and gradually form a harem around them. It also gives a chance for some cute and comical interactions, like when the characters are all eagerly waiting for the OC's next meal.