Maybe there's an evil spirit that has a grudge against the dragons. When the sister starts to get involved with the dragons, this spirit targets her. It could be a spirit that was once wronged by the dragons in the past and now wants revenge, not caring that the sister is just a new acquaintance to the dragons.
Well, perhaps she could form a special bond with the dragons. Maybe she learns to ride them and uses their power to help Aang in his adventures. Or she might discover some long - lost secrets about the dragons that could change the course of their world.
She might form a special bond with the dragons. In many fanfictions, characters who meet dragons often develop a deep connection, either through shared magic or mutual understanding.
It could be Frieza. Since Frieza has a history of hating the Saiyans, he might see Goku's sister as a new threat and try to eliminate her.
It could be someone who is jealous of their new - found bond. Maybe a rival of Uzumaki who doesn't want him to have such a powerful ally in his sister.
It could be someone who doesn't want Naruto and his sister to unite. Maybe a rogue ninja who fears their combined power. He might try to drive a wedge between them by spreading false rumors or causing misunderstandings.
It could be Zuko if the fanfiction takes a different turn. Maybe Zuko, still struggling with his own inner demons, sees Aang's loss of control as an opportunity to take advantage and gain power. So he tries to manipulate the situation to his benefit.
Morgana could be a great antagonist. She might see Merlin's sister as a threat to her own plans for Camelot. Maybe she discovers that Merlin's sister has a power that could undermine her magic or her influence over the court.
Well, it could be a magical encounter. She might be in awe as she first lays eyes on the magnificent dragons. Maybe they approach her gently, their scales glistening in the light.
A powerful dark dragon could be the main antagonist. This dragon might be jealous of the little flame dragon's unique abilities or position within the dragon community. It could constantly try to undermine the little flame dragon and cause trouble for it.
It could be a powerful dark wizard who wants to control the dragons and sees Naruto as a threat because of his newfound dragon - trained powers.