An ancient evil entity from Cybertron's past could be the antagonist. It awakens and wants to use the Cybertronians for its own dark purposes. Harry, leading the group, has to find a way to stop this entity before it's too late. This entity could have powers that are both technological and magical in nature, making it a formidable foe.
The plot could involve Harry leading the Cybertronians in a battle against an evil force that threatens both their worlds. They might use their unique abilities - Harry with his magic (if he has some in this fanfic) and the Cybertronians with their advanced technology.
The Ministry of Magic could be the antagonists. Since Harry is in a gang, they might see him as a threat to the order of the wizarding world. They could send Aurors after him and his gang mates, trying to arrest them and break up the gang.
It could be rogue ninja groups that oppose Naruto's new way of ruling. These groups might believe that Naruto is trying to take away their freedom and the traditional ways of the ninja world. They could try to undermine his empire by causing chaos and attacking his infrastructure.
The Thalmor could be great antagonists. Their strict control over magic and their anti - human beliefs could put them at odds with Harry. They might see his different form of magic as a threat to their power in Skyrim.
Some of the more traditional pure - blood families in the wizarding world could be antagonists. They might not approve of Harry, a half - blood, being involved with Pansy, and they could try to stop the pregnancy or break up the couple for the sake of 'pure - blood' ideals.
Voldemort or his followers could still be antagonists. They might not like the idea of Harry being under Euryale's protection as it could make him more powerful. Another possibility could be some creatures from Euryale's world who are against her taking in an outsider like Harry.
Perhaps the very magic that made Harry a god has a sort of negative side - effect. This could manifest as a shadowy entity within Harry that tries to take over his mind and use his powers for evil. So, in a way, this internal struggle could be the main antagonist in the story.
It could be other members of the royal family who see Harry as a threat to their power or position. They might try to undermine him and prevent him from claiming his rightful place as king.
It could be a new dark wizard who wants to use Harry's daughter to get revenge on Harry. This wizard might be jealous of Harry's fame and power and think that by harming his daughter, he can destroy Harry's happiness.
One potential main antagonist could be a powerful pirate captain who sees Harry as a threat to his control over the seas. Maybe this captain has some dark magic powers as well.