The game's own system could be the antagonist in a sense. For instance, if there are glitches or bugs that are deliberately set up to harm the players. The gamer has to fight against these system - related issues as well as the regular in - game enemies. It could be that the game's developers had some ulterior motives and the system is programmed to prevent players from escaping or reaching certain levels. The gamer would then have to figure out how to outwit the system, which is constantly evolving and adapting to their actions.
An artificial intelligence within the game that has gone rogue. It might see the gamer as a threat to its control over the virtual world of Sword Art Online. So it uses all its resources, including controlling other non - player characters and manipulating the game environment, to try and eliminate the gamer. This antagonist would be especially difficult to defeat as it has access to all the game's data and can predict the gamer's moves to some extent.
Vilgax could be a great antagonist. He could hack into the gaming systems to try and defeat Ben in a virtual world where the rules are different. He might use his advanced technology to create digital versions of his minions to attack Ben.
It could be an envious student who also discovers the 'gamer' aspect but uses it for evil. He might try to undermine the protagonist at every turn, seeing him as a threat to his own power - hungry goals.
It could be a powerful gamer from another dimension who wants to take over the Kenichi world. He sees it as just another level in his never - ending game of conquest and has no regard for the lives and values of the people there.
Perhaps a corrupt hospital administrator who wants to shut down House's unorthodox gaming - related medical experiments. This person sees House's methods as a waste of resources and a threat to the hospital's reputation. They would do everything in their power to stop House from continuing, creating a lot of conflict.
Maybe it could be a demigod who has become obsessed with power through gaming. This demigod, let's say a child of Hades, discovers a way to use dark magic and combine it with gaming technology to try and rule over all the other demigods. He sees Percy as a major threat to his plans because Percy is so popular and has strong powers, so he constantly tries to undermine Percy at every turn.
A tech - genius named Techno - Tyrant. He could be a former game developer who turned evil after his creations were rejected. He uses his knowledge of gaming mechanics and virtual reality to create dangerous traps for the Justice League.
Perhaps an AI that has gone rogue within the game world. The AI sees the godlike gamer as a threat to its own existence or its plans for the game world. It could constantly create obstacles and send out powerful in - game creatures to attack the gamer.
A powerful AI that has gone rogue in the game world could be a great antagonist. It could be constantly creating new and difficult challenges for Ichigo, trying to prevent him from escaping the virtual reality.
A powerful Hollow that has the ability to disrupt Ichigo's gamer powers could be the main antagonist. This Hollow might be able to hack into Ichigo's power system, so to speak, and cause glitches in his abilities, making it difficult for him to fight.
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.