Who could be the main antagonist in 'oc reincarnated into pjs fanfiction'?

One idea could be that the oc retains some memories of their past life. So they use their knowledge to change things in the pjs world. For example, they might know of a coming danger and work to prevent it.

It could be an old enemy from Percy's past life who has also found a way to be reincarnated. This enemy might hold a grudge against Percy for something that happened in their previous encounter and is now seeking revenge in this new life.

Maybe there is a new character created specifically for this fanfiction as the main antagonist. This new character could have a connection to the oc's past life or the reason for their rebirth. They might be trying to prevent the oc from fulfilling a certain destiny or unlocking their full potential.

It could be another powerful ninja who sees the student as a threat to their own power or plans. Maybe someone who doesn't want Madara to pass on his knowledge to an outsider or a young upstart.

An ancient and powerful entity that has awakened from a long slumber. It could be some sort of dark psionic being that wants to consume all life in the galaxy, starting with the Protoss and the OC's army.

Another possibility could be an original antagonist created specifically for the fanfiction. For example, a powerful sorcerer from a hidden dimension who sees the OC's rebirth as a disruption to the natural order of the MCU universe. This sorcerer could try to eliminate the OC and use their power for their own dark purposes. They might be able to manipulate magic in ways that even Doctor Strange hasn't seen before, making them a formidable foe.

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 someone who was an enemy of Molly in his past life and now sees his reincarnation as a threat.

Maybe a new character could be introduced as the main antagonist in the fanfiction. This character could be a powerful knight from another region who has a personal vendetta against Lelouch or the OC. They might have different beliefs about how the world should be ruled and see Lelouch's actions, and by extension the OC knight's support, as a threat to their vision of order.

It could be another powerful character who is jealous of the growing relationship between Copperhead and the male OC. This character might see the male OC as a threat to their own power or influence over Copperhead. For example, if Copperhead is a leader in a certain group, this antagonist could be a rival within the group who wants to undermine Copperhead by getting rid of the male OC.