Madara Uchiha could be a great choice for the main villain. Even if he's been defeated before, in this fanfiction, he could be resurrected with new powers. He would see the Naruto - Sasuke merge as an obstacle to his plan of reshaping the ninja world according to his will.
The main villain could be a new character. Maybe a rogue ninja who has been researching the secrets of power fusion for years. He sees Naruto and Sasuke's merge as the perfect specimen to study and will stop at nothing to capture them, even if it means destroying the entire ninja world in the process.
It could be Orochimaru. He might be jealous of their new merged power and see it as a threat. So, he tries to find a way to either steal it or destroy them.
One possible plot could be that Naruto and Sasuke, after many battles and a deep understanding of each other's pain, find a way to merge their powers. This new merged form could be used to defeat a greater evil that threatens the ninja world. They might have to sacrifice some of their individuality but gain new abilities that neither of them had before.
A new character could be created as the main villain. Maybe a rogue ANBU agent who was once a friend of Naruto and Sasuke but has been corrupted by power. This character would know all the ins and outs of the ANBU, making it very difficult for Naruto and Sasuke to stop him as he tries to take over the ninja villages using the secrets he learned while in the ANBU.
It could be an unknown, ancient evil that has been awakened. This villain might have powers far beyond what Naruto and Sasuke have faced before. Maybe it's an entity from a forgotten era that was sealed away long ago and now has broken free.
In this kind of fanfiction, the main antagonist could be a new character created out of the chaos caused by the merge. For example, the merge could disrupt the balance of the ninja world in a way that awakens an ancient evil force that was sealed away long ago. This force would see the merged Naruto - Madara as a target to destroy because they represent a new power that could oppose it.
One main theme is identity. When Naruto is in Sasuke's shoes, he has to grapple with who he is now. Another theme is friendship. Naruto might view his relationships with others differently as Sasuke. And there's also the theme of power. Sasuke has a different power level and Naruto has to adapt to that in these fanfictions.
The merge can also happen on a more conceptual level. For example, the idea of friendship and teamwork, which are important in both series, could be the connection. Naruto's belief in his friends and the Digidestined's bond with their Digimon could be seen as similar. This similarity could be the starting point for the two worlds to interact in fanfiction. Writers might have Naruto and the Digidestined team up to fight a common enemy that threatens both the Digital World and the ninja world. They would combine their unique powers and skills to achieve victory.
The main characters are Naruto and Sasuke, obviously. They are two of the most iconic characters from the Naruto series. Naruto is known for his exuberant and determined personality, while Sasuke is more brooding and focused on his own goals, and in this fanfiction, their relationship in the context of Naruto 'riding' Sasuke is what the story likely revolves around.
One of the main themes is friendship. Their relationship is complex, and fanfictions often explore how their friendship endures through all the challenges they face. For example, in many stories, Naruto's unwavering belief in Sasuke is a central point.
There could be an outside force, like a neighboring village that wants to keep Naruto and Sasuke away from their own village. They might be afraid of the potential power and influence Naruto and Sasuke could bring back if they return. So, this neighboring village sends their best ninjas to hunt down Naruto and Sasuke during their banishment, making them the main antagonists in the fanfiction.
While Naruto and Sasuke are the main focus, other characters could also play important roles. For instance, Sakura might visit them, which could create some interesting dynamics. Kakashi could also drop by to check on them and offer his guidance. But overall, it's Naruto and Sasuke's relationship that will be at the heart of the story.