Another possibility is a group of Hogwarts professors who are very conservative in their views. They might believe that house elves should remain in their subservient positions and would try to stop Harry from making any changes. They could use their influence within the school to make things difficult for Harry and her friends as they try to advocate for the rights of house elves.
Maybe Harry Potter, as a female, could start to understand the struggles of house elves deeply. She might use her magic to try and improve their living conditions, like Hermione did with the 'Society for the Promotion of Elfish Welfare'. She could also form unique friendships with some house elves, leading to adventures within Hogwarts and beyond as they work together to change the traditional views of house elves in the wizarding world.
It could be a powerful wizard who sees Harry's transformation as an abomination and wants to destroy him. This wizard might believe that Nagas are dark creatures and that Harry, by becoming one, has crossed a line into the dark side of magic.
There could be a group of pure - blood wizards who believe that dragons are beneath them and that Harry is tainting the wizarding world by associating with them. They would try to stop Harry from his dragon - related activities, and they might use underhanded tactics like casting spells on him or trying to get him expelled from Hogwarts.
In some cases, it could be other dark wizards or witches who are envious of Harry's new power. They might want to steal his Obscurial power for themselves or use it to gain dominance in the dark wizarding underworld. These antagonists might be minor characters in the original series but could be developed more in the fanfiction to create new and interesting conflicts for Harry.
It could still be Voldemort. He might see Harry as an abomination as a cyborg, a perversion of the pure wizard blood. He would then be more determined than ever to destroy Harry, fearing that his new form gives him an edge in the fight against the dark arts.
Well, perhaps Harry could start seeing the world from the house - elves' perspective. He might use his magic in new ways to improve their living conditions. For example, he could secretly create better living quarters for them in Hogwarts. He might also try to teach them some basic self - defense magic so they can protect themselves from the meaner wizards.
It could be a dark wizard who wants to take over the toy house for its magical properties. Maybe he believes that there are powerful artifacts hidden within the toys that can make him invincible.
Perhaps some of the pure - blood wizard families who look down on house elves training a wizard like Harry could be antagonists. They might see it as a threat to the traditional wizarding hierarchy and try to stop Harry and the house elves, thinking that it's not proper for a wizard of Harry's status to be trained by 'mere' house elves.
The Ministry of Magic could be the main antagonist. They might view Harry's fascination with Dementors as a sign of him turning to the dark side. So they try to stop him from his research and exploration of Dementors, fearing he'll use their powers for evil. After all, Dementors are associated with dark magic in the eyes of the Ministry.
There could also be new antagonists. For example, some of the more traditional wizarding families who are against the idea of a female Harry. They might see her as a threat to the old - fashioned ways of the wizarding world and try to undermine her.