Maybe some evil wizards or dark creatures. They could sense the power of the growing dragon and want to either control it for their own evil purposes or kill it to prevent it from becoming a threat to their plans. They would see Bilbo as an obstacle in their way.
There could be other dragon - slayers in the story. These are people who have a long - standing hatred for dragons and believe that all dragons should be killed. They would see Bilbo's act of raising a dragon as wrong and immoral, and would stop at nothing to put an end to the dragon's life, making them antagonists in this fanfiction.
In this fanfiction, perhaps Bilbo discovers a dragon egg. He decides to raise the dragon. He might face challenges like hiding it from the dwarves at first. As the dragon grows, Bilbo has to teach it right from wrong, which is a huge task considering dragons are known for their wild nature in Middle - earth lore.
Obviously, the growing Orcs themselves could be the main antagonists. Their increased size and strength make them a greater threat to Bilbo and the other good - natured races in Middle - earth.
One possible antagonist could be the time itself. Since Harry is messing with the natural flow of time, there could be some sort of time - based magic or entity that tries to stop him. It might be a time - keeper or some ancient magic that protects the integrity of the timeline.
It could be a powerful dragon that has claimed the treasure as its own and will stop at nothing to protect it.
There could be an old dragon that has been in exile for a long time. This dragon might see Hiccup as a threat to its plan of taking over the dragon kingdom. So it tries to stop Hiccup from fully becoming a dragon or tries to use his new form for its own evil purposes.
Blackbeard could also be a main antagonist. He has his own ambitions for power and the One Piece. With Luffy getting stronger under Dragon's training, Blackbeard would view Luffy as a direct competitor. Their paths are bound to cross, and Blackbeard would not want Luffy to achieve his goals before he does.
Another option is a dark cult that worships chaos. They believe that the dragon child's destiny to bring balance will disrupt their plans for a world in anarchy. So, they set out to corrupt the dragon child or kill it to keep their vision of chaos alive.
It could be some of the more powerful villains from both universes combined. Maybe Frieza and Orochimaru team up. Frieza's lust for power and Orochimaru's desire for immortality and knowledge would make them a formidable pair. They would target Naruto because of his potential and Goku because of his strength, aiming to either destroy them or use them for their own evil purposes.
It could be other villains who are jealous of Izuku's powerful dragon quirk. They might see his quirk as a threat to their own plans for domination or chaos.
It could be a group of rogue demigods who want to use the Dragon King's power for their own evil purposes. They might be jealous of Percy's status and see the Dragon King as a means to gain more power and control in the mythical world.