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.
Another antagonist could be a jealous Viking who has always been envious of Hiccup's connection with dragons. When Hiccup becomes a dragon, this Viking spreads rumors among the people that Hiccup is now a dangerous monster. This causes a lot of problems for Hiccup as he tries to adjust to his new form.
A group of dragon hunters who don't believe in Hiccup's divinity could be antagonists. They think that dragons should be under their control, not Hiccup's. So they set out to capture all the dragons and overthrow Hiccup. They might use new and dangerous weapons to achieve their goals.
Well, it could be that Hiccup is cursed by an evil sorcerer who wants to turn him into a dragon as a form of punishment. But as he turns into a dragon, he finds that he has a new connection with the dragon world. He starts to see things from a dragon's perspective and uses this to bring peace between humans and dragons in a whole new way.
One possible antagonist could be a jealous sorcerer. He might be envious of Hiccup and Elsa's relationship and the power that their unborn child is said to possess. So he tries to harm Elsa during her pregnancy to stop the child from being born. Another could be some of the traditionalists from their respective kingdoms. They don't approve of their relationship and see the pregnancy as a threat to their old ways. They might try to separate Hiccup and Elsa, which would put Elsa's pregnancy at risk.
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.
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.
The Hidden Leaf Village elders could be antagonists. If Naruto turns dark, they might see him as a threat to their power and try to stop him. Their actions against Naruto could further fuel his descent into darkness.
Morgana could be a main antagonist. She has her own desires for power and may see Merlin's rise to kingship as a threat to her plans. So she tries to stop him at every turn, using her dark magic against him.
The Titans could be the main antagonists. They might oppose Percy's ascension to Olympian status because they see it as a threat to their own plans of overthrowing the Olympians.
Some jealous students at Hogwarts could also act as antagonists. They might be envious of Harry's newfound allure and use underhanded tactics to try and make his life difficult.
The Hidden Leaf Village itself could become the main antagonist. Since Naruto turns evil, the village that once nurtured him would now be against him. The village elders and some of the ninja who oppose Naruto's new evil ways would be in conflict with him. For example, Kakashi might be forced to fight against Naruto to protect the village.