An ancient evil that has been awakened. It could be a powerful sorcerer from the past who wants to control all the dragons for his own dark purposes. He might use magic to manipulate the dragons and turn them against Hiccup and his friends.
A rogue dragon rider who has different ideas about how to treat dragons. He might see them as tools for power rather than friends like Hiccup does. This rider could be causing trouble for Hiccup and his tribe by stealing dragons or using them to attack other tribes.
A rival tribe that is jealous of Hiccup's bond with the dragons. They believe that the dragons should be under their control and they start a conflict with Hiccup's tribe. Their leader could be the main antagonist, constantly devising plans to undermine Hiccup and take the dragons for themselves.
It could be a dragon hunter who sees Hiccup in his dragon form and wants to capture him for study or to use his unique abilities for evil purposes.
A jealous rival who also wants to be known as the dragon master. This person would try to undermine Ash at every turn, spreading false rumors about him and trying to steal his dragon - taming techniques.
A powerful dragon rider from a far - off land who has a grudge against the Berk tribe (Hiccup's tribe) could be the main antagonist. He or she sees Hiccup and Heather's marriage as a symbol of the growing strength of Berk and decides to target them specifically to weaken the tribe. They might try to steal their dragons or spread false rumors about their marriage to create unrest among the people.
A new dragon - hunter could be the main antagonist. He might be extremely skilled and ruthless, and Hiccup hides his strength to better understand this new threat and find a way to defeat him without putting his village in immediate danger.
Maybe a jealous rival from within the village. This person has always been envious of Hiccup's relationship with Toothless and his status as a dragon trainer. When Hiccup gets struck by lightning, this rival spreads rumors that Hiccup is now cursed and should be ostracized. He tries to turn the villagers against Hiccup in order to take his place as the leading figure when it comes to dragons in the village.
Maybe a rival Viking tribe that has a different view on dragons. They see dragons as a threat rather than friends like Hiccup and Valka do. So they try to disrupt the peace that Hiccup and Valka are trying to build between the Vikings and the dragons. They could be constantly causing trouble, stealing dragon eggs or attacking dragons, which would force Hiccup and Valka to take action against them.
A power - hungry Viking who sees Hiccup's transformation as a threat to his own authority could be the main antagonist. He might try to capture Hiccup in his Night Fury form and use him for his own evil purposes, like trying to create an army of transformed creatures.
The main antagonist could be a jealous character who is envious of Bertha's unique features, like her 'big boob'. This character might try to use Bertha's hiccup problem against her, perhaps by making her hiccup in embarrassing situations to lower her self - esteem.
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.
It could be a rogue ninja who also has strong water - based abilities and wants to prove that he is the true master of water. He might see Naruto's newfound skills as a threat.