Another option could be an evil version of Android 17 or 18. They could be corrupted in some way and oppose both Cell and the OC. They might have their own plans for world domination and view Cell and the OC as obstacles in their path.
Frieza could be a great antagonist. He might see Cell's power as a threat and try to eliminate him while the OC is caught in the middle. Frieza could also try to use the OC to his advantage against Cell, creating a complex web of power struggles.
It could be Beerus. Since he is a powerful God of Destruction, he would see the union of Frieza and Cell as a threat to the balance of the universe and try to stop them.
It could be someone from Goku's past who has a vendetta against him and takes advantage of the chaos during the Cell Games. For instance, an old enemy who was thought to be defeated but managed to come back stronger and wants to finish Goku off while he's weakened from the fight with Cell.
Perhaps a dark elf from the elf OC's world who has turned bad. This dark elf could be jealous of the elf OC's connection with Naruto and the Naruto world. So, he follows the elf OC to the Naruto world and tries to disrupt their friendship and cause chaos. He might have some dark magic that is a threat to both Naruto and his friends as well as the elf OC.
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.
Maybe the OC is a time - traveler who has come to prevent Cell from reaching his perfect form. They try to disrupt his plans at every turn. This could involve the OC teaming up with the Z - Fighters in some ways, and there would be a lot of action and strategy involved in their attempts to outwit Cell.
Mr. Sinister could be a great antagonist. He might be interested in the OC's unique genetic makeup, which could be related to Jean and Scott's powers in some way. He could try to manipulate the situation to his advantage, perhaps by pitting Jean and Scott against each other over the OC, or by trying to turn the OC against them.
A power - hungry scientist who wants to use patient zero OC's unique condition for their own unethical experiments. They see the OC as a tool rather than a person and will stop at nothing to get what they want.
One option for the main antagonist could be a powerful sorcerer who wants to harness the Oc Hulk's power for his own dark magic. He might see the Oc Hulk as a key ingredient to a spell that would give him ultimate power over the world. The sorcerer could use his magic to create illusions and traps for the Agents of Smash, making it difficult for them to protect the Oc Hulk.
A powerful cult leader among the humans could be a great antagonist. They might see the godlike OC as a threat to their power and influence. They could spread lies about the OC, turning the people against them, and even try to assassinate the OC to maintain their control over the walled cities.
One popular 'Cell x OC fanfiction' is 'Cell's New Beginning'. It explores a unique relationship between Cell and an original character. Another good one could be 'Cell and the Mysterious OC' which has an interesting plot that involves Cell's power and the OC's special abilities.