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.
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.
Some dragons are more aggressive by nature. In fanfics, these dragons might be more likely to attack any potential threat to Hiccup without hesitation. For example, a dragon like Hookfang might be quick to breathe fire at anyone who gets too close to Hiccup in a menacing way. On the other hand, a more gentle dragon like Meatlug might use her size to block Hiccup from harm rather than being as confrontational.
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 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 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.
One idea could be that Hiccup discovers a new type of dragon with unique abilities. For example, a dragon that can change its color to blend in with its surroundings. This could lead to a whole new adventure as he tries to understand this dragon and protect it from those who might want to use it for bad purposes.
In a particular 'Dragons Protect Hiccup Fanfic', the dragons' unique abilities play a big role. For instance, some dragons might have the ability to become invisible. They use this to spy on threats to Hiccup and then protect him when the time comes. There could also be a fanfic where the dragons can communicate with each other over long distances and they coordinate their efforts to form a protective perimeter around Hiccup. It's all about how the author imagines the dragons using their skills to keep Hiccup out of harm's way.