A new demon that thrives in dark and stormy conditions could be the main antagonist. This demon could be attracted to the negative energy of the storm and decides to target Inuyasha. It might have unique powers, like being able to control the lightning in the storm to attack Inuyasha.
The plot could involve Inuyasha being in a dangerous situation during a dark and stormy night. Maybe he's facing a powerful new enemy that emerges under the cover of the storm. The storm could also be a magical creation by an evil force to hinder Inuyasha's movements.
It could be a powerful ninja who has become corrupted by the idea of the descendant's powers. He wants to steal those powers for himself to gain ultimate control over the ninja world. Maybe he has a deep - seated envy of the unique combination of abilities the descendant has.
It could be a powerful ninja - demon hybrid. This antagonist could be jealous of the bond between Naruto and Inuyasha. He wants to use their combined power for his own evil purposes. So, he tries to turn them against each other first by spreading false rumors and then by attacking their loved ones.
Perhaps another reborn character could be the main antagonist. Say, an evil sorcerer from the past who was also reborn in the Inuyasha world at the same time as the OC. This sorcerer has his own agenda of ruling the world and sees the OC as a threat to his plans, especially if the OC is getting in the way of his attempts to gather the Shikon Jewel fragments.
It could be a powerful ninja from Naruto's world who has been corrupted by the Juubi's power and somehow ended up in the Inuyasha world. He wants to use the power of the demons in the Inuyasha world to make himself even stronger and take over both worlds.
Perhaps a rogue vampire who has his own agenda. He might be jealous of the red knight's power or status and constantly tries to undermine them. This vampire could be very cunning and not afraid to use dirty tricks to get what he wants.
Maybe the Akatsuki as a whole. They are a group of powerful and dangerous ninjas. The Black Knight's presence might disrupt their plans to collect all the tailed beasts. They would see him as an obstacle and try to eliminate him. For example, Pain might think the Black Knight is a threat to his vision of peace through pain and send his subordinates to fight him.
It could be a powerful and corrupted Huntsman. He might have once been a colleague of the Silent Knight but was consumed by greed for power. He now uses his skills to create chaos and try to take over the kingdoms, and the Silent Knight has to stop him.
It could be Orochimaru. In the medieval knight setting, he might be a dark sorcerer who wants to use the power of the knights for his own evil purposes. He could be trying to steal the holy relics of the kingdom which are said to grant great power, and Naruto, as a knight or someone with knight - like qualities, has to stop him.
Perhaps a group of dark side cultists could be the main antagonists. They idolize the power of the dark side and see Ahsoka as a prime candidate to join their ranks. When she refuses at first, they start to hunt her down, but in the process, they unknowingly push her closer to the dark side, creating a complex dynamic where they are both the pursuers and the inadvertent cause of her fall to the dark side.