There could also be a group of humans who are jealous of Hiccup's newfound connection with the dragons. They might be from another village and feel that Hiccup is getting special treatment. So, they plot to drive a wedge between Hiccup and the dragons and take over Berk for themselves.
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, the plot could involve Hiccup discovering his new role as the Prince of Dragons and having to deal with the responsibilities that come with it. He might have to unite different dragon tribes under his leadership and face challenges from dragon hunters who see his new position as a threat.
One possible antagonist could be a jealous noble from Naruto's own kingdom who feels that Naruto doesn't deserve the prince title. This person might try to undermine Naruto at every turn, spreading false rumors about him or trying to sabotage his diplomatic efforts.
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.
The dragons themselves are also important characters. There could be Toothless, who is often Hiccup's closest dragon companion in the original stories. And there might be other dragons like Stormfly, Hookfang, etc., each with their own personalities and roles in Hiccup's upbringing.
The Titans could still be a major threat. Even though they were mostly defeated in the original series, some of their followers might see Percy's new role in Atlantis as a threat to their plans for chaos. They could try to undermine Percy's position in Atlantis and use the underwater city as a base for a new uprising.
Obviously, Hiccup is one of the main characters since the title mentions him. He is likely to be the central figure around which the story revolves.
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.
A young dragon rider from a distant island who has a very different style of riding and communicating with dragons. This character could bring new techniques and ideas to Hiccup and his friends.
It could be a group of rogue ninjas who want to capture the dragons for their power. They might be led by a powerful and greedy ninja who sees the dragons as a means to gain ultimate power over the ninja world.