There could also be some dark wizards who sense the power of the One Ring. They might team up with Voldemort or work on their own to get the Ring from Harry. These dark wizards could be those who have always been on the fringes of the wizarding world, looking for a chance to gain ultimate power.
Sauron, even in the context of this fanfiction crossover, could be an antagonist. His influence could reach the wizarding world through the One Ring. He might try to manipulate Harry or those around him to get the Ring back.
Definitely Voldemort and Sauron. Voldemort would see the One Ring as a powerful tool to conquer the wizarding world completely. Sauron, on the other hand, would not want anyone else to have control over the ring. So they would both be after Harry and the ring, making them the main antagonists.
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.
Voldemort could still be a main antagonist. He might be especially interested in Harry's Sacred Gear because it could potentially give Harry an edge over him in their battles.
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.
Perhaps the One Ring has some sort of connection to Voldemort. Harry, having the One Ring, finds that Voldemort is drawn to it because of its power. This could lead to a new battle where Harry not only has to face Voldemort but also has to decide what to do with the Ring. He might have to journey to find a way to destroy it, just like in the original 'Lord of the Rings' story.
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.