Who could be the main antagonist in an 'oc shinshi in west blue fanfic'?

The 'oc shinshi in west blue fanfic' might revolve around Shinshi being a marine trainee sent to the West Blue for some special mission. He could encounter a strange and powerful pirate crew that he has to either defeat or make an uneasy alliance with to complete his mission. His personal growth and the relationships he forms in the process would be key elements of the story.

A strict teacher at Rydell High could be the main antagonist. The teacher might disapprove of the OC's influence on Michael, thinking that the OC is distracting Michael from his studies. The teacher could constantly be giving them detention or trying to keep them apart, which would create conflict in the fanfic.

A new, original villain created just for this fanfic could be the main antagonist. This villain could be a powerful sorcerer who has been observing the Avengers from the shadows. When the OC is reborn, they sense a disruption in the magical balance of the world. Believing the OC to be a threat to their own plans of ruling the world through magic, they set out to eliminate the OC and in the process, come into conflict with the Avengers.

It could be a powerful Arrancar. In the Bleach universe, Arrancars are often antagonists. A particularly strong and cunning one might see the oc as a threat to their plans. Maybe the oc's presence disrupts the Arrancar's scheme to take over a certain part of the Soul Society or gain more power.

A powerful rogue gem who has been stealing energy from other gems and growing stronger could be the main antagonist. This gem could be jealous of Blue Diamond and Yellow Diamond's power and status and aims to take them down.

Perhaps an existing 'One Piece' character could be turned into the main antagonist in the context of this fanfic. For instance, a marine officer who sees the OC as a threat to the world government's order. Since Viola has ties to Dressrosa, which has its own complex relationship with the world government, this marine could target them both. The officer might use underhanded tactics to try and stop the OC and Viola from achieving their goals.

A powerful dark mage who was once a member of Fairy Tail but was banished for using forbidden magic. This mage now wants revenge on the guild and sees the reborn OC as a threat to his plans. He has the ability to control shadows and can create illusions to deceive the members of Fairy Tail.

It could be a Rebel spy within the Empire. This spy would be constantly trying to undermine the OC's progress and expose their true intentions. The spy might see the OC as a threat to the Rebellion's plans, especially if the OC is rising in the ranks quickly.

A human hunter could be the main antagonist. He has a grudge against all kung fu animals, especially wolves. He believes they are too powerful and dangerous. He hunts them down one by one and when he hears about the wolf OC in the Valley of Peace, he sets his sights on the valley, causing chaos and danger for all the inhabitants, including the wolf OC and his new friends.

It could be a powerful western ninja who doesn't like Naruto encroaching on their territory. They might see Naruto as a threat to their power and way of life.