There could also be some original characters created specifically for this fanfiction. For example, a powerful ninja who has knowledge about the merge process and tries to either help or hinder Naruto. Or perhaps an ancient spirit that was somehow involved in the creation of Dark Naruto in the first place and now has an interest in the merge.
Perhaps in the fanfiction, the merge between Naruto and Dark Naruto is caused by a powerful jutsu gone wrong. As a result, Naruto has to struggle to control the darker impulses while also harnessing the newfound power. This internal conflict could lead to some really engaging story arcs where he battles both external foes and his own inner demons.
One way could be through a magical portal. Imagine in a fanfic, a powerful mage in the Game of Thrones world accidentally opens a portal to the Naruto universe. Dark Naruto, with his edgy and powerful nature, could be seen as a threat similar to some of the dark forces in Game of Thrones. He might form an alliance with a power - hungry lord in Westeros, thinking he can use their resources to achieve his own goals in the Naruto world. For example, if he teams up with Littlefinger, they could plot to take over both kingdoms using Dark Naruto's powers and Littlefinger's cunning.
If Naruto merged with Yami Naruto in fanfiction, he could potentially access a whole new level of chakra. Yami Naruto has a different kind of energy, and when combined with Naruto's own chakra, it might create a more powerful and unique form of energy. This could give him an edge in battles, allowing him to defeat enemies that he previously couldn't. Also, his appearance might change, taking on some of the more menacing features of Yami Naruto.
In 'Naruto merged with his dark side fanfic', a prevalent theme is the battle between good and evil within Naruto himself. He has to decide how much of his dark side he will let influence his actions. Also, redemption is often a theme. Even though he has merged with his dark side, he may still strive to be a hero in the end. Additionally, the theme of acceptance by others is important. His friends and the village need to come to terms with this new Naruto, which can lead to a lot of drama and character development.
The fanfiction could change the Naruto universe by introducing new threats. With Naruto so powerful, there could be forces that emerge to try and counterbalance his strength. Also, the story could explore different ways of using chakra, as Naruto's merged state might have unique chakra abilities. It could also affect the way ninjas are trained, as they might try to achieve a similar level of power.
The story could progress with Naruto facing new and more powerful enemies. Since he has merged with Yami Naruto, his power level has increased, and this would attract stronger foes. He might also go on a journey to better understand and control his new powers.
A more complex way to merge the worlds is to have a common enemy. For example, a new and extremely powerful dark force that threatens both the ninja villages in Naruto and the realms in Castlevania. This would force the characters from both worlds to work together. They would have to share their knowledge and powers. In this scenario, Naruto could teach the Castlevania characters about chakra, while they could teach him about dark magic defenses. It would be a story full of action and discovery as the characters adapt to each other's worlds and powers.
In such a fanfiction scenario where Naruto merged with Madara, their chakra would blend in an unprecedented way. Naruto's vast chakra reserves combined with Madara's Uchiha blood and powerful chakra would make them an almost god - like entity. Their merged form could have abilities like instant teleportation across great distances, the power to nullify any ninjutsu with a simple thought, and they could also potentially reshape the very fabric of the ninja world, creating a utopia where all ninja clans live in harmony, using their combined knowledge and power to ensure there is no more war or hatred.
It could be the remaining Otsutsuki clan members. Since Naruto has merged with Kaguya, they might see this as a threat to their plans for the ninja world or a perversion of their bloodline. They could try to either eliminate the merged Naruto - Kaguya or control it for their own purposes.