Perhaps it could be a member of the Donquixote Family itself who has gone rogue and wants to take over the family completely. This character could see Law as either an obstacle or a tool to achieve their goal. They might try to frame Law for crimes against the Donquixote Family to gain the loyalty of the other members and at the same time, eliminate any potential threats from Law's side. This would create a complex situation where Law has to clear his name while also dealing with the internal power struggle of the Donquixote Family.
The main antagonist could also be a powerful figure from the World Government who sees the potential alliance or interaction between Law and the Donquixote Family as a threat. They might try to manipulate the situation to turn Law and the Donquixote Family against each other so that they can eliminate both threats at once. This antagonist could use false information, set up traps, and send in their own agents to disrupt any positive developments between Law and the Donquixote Family.
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.
One idea could be that Law has to team up with a member of the Donquixote Family against a common enemy. Maybe they start off distrusting each other because of their past associations but as they face challenges, they learn to work together. For example, Law and Baby 5 could be forced to cooperate when a new powerful pirate group threatens the territory that both have some sort of interest in.
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.
Perhaps an ancient being that was sealed away in both worlds long ago and is now awakened. This being could be extremely powerful, seeing the combined Naruto and One Piece universes as a threat or a source of power to be harnessed for its own dark purposes. It would be a formidable foe for Naruto and the One Piece characters alike.
It could be Orochimaru. He's always been interested in unique powers and abilities. The idea of X - 23's mutant abilities and Naruto's jinchuriki powers would be very appealing to him. He might try to capture them both to use in his experiments to gain ultimate power.
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.
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.
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.
It could be a powerful sorcerer from a magic - based crossover world. This sorcerer sees Ace's power as a threat to his own rule and plans to either capture Ace and use his power for his own evil deeds or simply destroy him.
There could also be a new pirate crew led by a captain who has a grudge against both Natsu and the Straw Hat crew. This captain might have a Devil Fruit power that gives him an edge in combat against Natsu's fire and the Straw Hats' unique abilities, and his goal could be to take over the Grand Line and eliminate any potential threats, which includes Natsu and the Straw Hats.
It could be a powerful pirate crew that doesn't like the idea of an outsider like Naruto coming into their territory. They might see him as a potential rival for treasures or influence, so they try to drive him out or even eliminate him, thus becoming the main antagonists.