Maybe the main antagonist is a group of rogue ninjas who have banded together. They believe that the old ways of the ninja world, which Rikudou Sennin represents, should be completely destroyed. So they target him and his allies in the fanfiction. They are constantly causing chaos and trying to undermine any attempts at peace and stability in the ninja world.
It could be a powerful dark ninja who has discovered a way to tap into an ancient and evil power source. This ninja wants to use this power to rule the ninja world and sees Rikudou Sennin's rebirth as a threat to his plans.
One interesting plot could be that Rikudou Sennin is reborn into a time when the ninja world is on the verge of a new, unknown threat. He has to quickly regain his powers and knowledge to guide the new generation of ninjas, like Naruto and his friends, to face this threat. Maybe he has lost some of his memories in the rebirth process, adding an element of mystery and self - discovery to the story.
It could be an ancient evil that Rikudou Sennin once sealed away. With Naruto and Rikudou Sennin meeting, this evil might sense an opportunity to break free and becomes the antagonist.
Yes, in some fanfictions, Naruto can be depicted as the Rikudou Sennin. Fanfiction writers often take creative liberties and come up with unique storylines where Naruto might inherit the powers or even be reincarnated as the Rikudou Sennin to explore different plot developments and character arcs.
One possibility is that Naruto has a spiritual journey with Rikudou Sennin. They could travel through different time periods related to the history of the ninja world. Naruto would see first - hand how the world came to be the way it is. And through this journey, Naruto's understanding of his own purpose and the ninja way could be deepened. Rikudou Sennin might also reveal some hidden prophecies or destinies that Naruto has to fulfill.
The main antagonist could be an ancient evil that was sealed away by Rikudo Sennin long ago. But due to some disruption in the fabric of space - time in the crossover, it gets released. This evil being has powers that rival Rikudo Sennin's and it wants to take over both the Naruto world and the world it was sealed in.
In fanfiction, the idea of Naruto being the son of Rikudou Sennin can be really interesting. It could change so many aspects of the story. For example, his relationship with other characters like Sasuke might be different. His power growth could also be explained in a whole new way. But we have to remember that this is just a fan - created concept and not part of the original Naruto story. The original story has its own well - established plot and character relationships that are different from this fanfiction idea.
One implication is that Naruto would have an extraordinary amount of power. He could potentially reshape the ninja world more easily. For instance, he might be able to end all wars and conflicts with a snap of his fingers.
It could be Naruto himself. Imagine if he was reborn as a sennin. He would have all his previous experiences and then the added power and wisdom of a sennin, which would make for an extremely powerful and interesting character to follow.
It could be a rogue ninja who has mastered the Chibaku Tensei technique and wants to use it to reshape the ninja world according to his own dark vision. He might see the current system as corrupt and decides to use extreme measures to create a new order.
One way they could interact is through a power - based scenario. Naruto's chakra and ninja skills, combined with the Six Paths Sage (Rikudou Sennin) powers, could be pitted against the superpowers of the Justice League members. For example, Naruto's speed could be compared to The Flash's. And the Sage's ability to manipulate chakra on a large scale might be equivalent to some of the more powerful Justice League members' energy - based attacks.