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.
Maybe it's a new pirate crew led by a character with a Devil Fruit power that can nullify the OC's Gamer abilities. This pirate captain is extremely ambitious and wants to rule the New World. He views the OC as an obstacle in his path because the OC has the potential to disrupt his plans, especially if they can use their Gamer skills to form alliances against him.
One interesting concept could be how the original 'One Piece' world is expanded with the addition of an original character (OC) related to Viola. Maybe the OC has unique abilities that interact with Viola's powers of seeing the truth. For example, the OC could have a power that distorts the truth in a way that Viola has to struggle to see through it, creating an interesting dynamic in the story.
One common theme is adventure. Since One Piece is all about adventure, these fanfics often have the OC and Viola going on adventures together, exploring new islands and facing challenges. Another theme is romance, of course. Writers like to explore the relationship between Viola and the OC, how they fall in love and deal with the obstacles in their relationship.
You can start by looking on fanfic platforms like Archive of Our Own (AO3) or FanFiction.net. These sites have a wide variety of fanfics, and you can use the search function to specifically look for One Piece Viola x OC stories. Just make sure to read the tags and summaries to find the ones that suit your taste.
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.
It could be a new character who has ties to both Law and the Donquixote Family in some way. For example, a former ally of Law who has now joined forces with the remnants of the Donquixote Family and has a vendetta against Law for some past betrayal. This character could use the resources and knowledge of both sides to create chaos and try to achieve their own goals, which might be something like taking over the underworld trade that both Law and the Donquixote Family have been involved in to some extent.
A powerful magical creature that has a grudge against both Harry Potter and Ace. It might see Ace as an intruder in its territory in the Harry Potter universe and Harry as someone who is interfering. So it tries to stop Harry from saving Ace and becomes the main antagonist.
Another option for the main antagonist could be a corrupted version of one of Whitebeard's own crew members. This character could have been influenced by some dark power and now wants to take over Whitebeard's position. They would know all of Whitebeard's weaknesses and strategies, making them a very formidable foe for the crew.