If Hiccup is under Pitch Black's control, it would be a huge shift in his relationship with Toothless. Normally, they have a deep, trusting bond. But now, Hiccup might start to command Toothless in a harsh and uncharacteristic way. Toothless would sense that something is wrong with his rider. He would try to resist Hiccup's commands at times, which would create conflict between them. Eventually, Toothless might be the key to breaking Pitch Black's control over Hiccup as he tries to reach the real Hiccup within.
One possible plot could be that Pitch Black uses his dark powers to manipulate Hiccup into turning against his friends in Berk. Hiccup, under the control, might start causing chaos, like setting dragons free to wreak havoc or stealing weapons from the village. Then, the story could revolve around his friends trying to figure out what's wrong with him and find a way to break Pitch Black's control.
Well, it would become a more equal relationship at first. Since Hiccup is now a dragon, he can understand Toothless on a whole new level. They might compete in dragon activities like flying races or hunting together. However, Toothless might also be a bit confused at first, not sure how to act around his once - human friend who is now a dragon.
His relationship with Toothless would become more complex. They would now be of the same species, which could lead to either competition or a deeper bond. Maybe Toothless would initially be confused about Hiccup's transformation but then accept him as an equal. They might communicate in new ways, without the need for Hiccup's human - made gadgets.
If Hiccup had powers, their bond could become even stronger. For example, if Hiccup could share his thoughts directly with Toothless using his new power, they would be able to coordinate in battles more seamlessly. Toothless might also become more protective of Hiccup, knowing that his rider now has powers that could attract unwanted attention. They could communicate in ways that no other dragon - rider pair could, like sharing memories or feelings instantaneously.
Their relationship would likely become more tender. Hiccup would be more protective of Astrid, always making sure she is safe and comfortable. Astrid, on the other hand, might be more reliant on Hiccup during her pregnancy.
There may be some extremely creative and boundary - pushing fanfiction out there with this premise, but it's not something that is typically found in the more well - known or widely - accepted "How to Train Your Dragon" fanfiction. Usually, fanfictions in this fandom focus on things like their adventures, friendship, and the development of their bond in a more traditional sense. So, while it's possible, it's not a common plot line.
Their relationship might become more equal in some ways. As dragons, they are both facing a new and strange existence, so they have to rely on each other more. They might develop a new understanding of each other's strengths and weaknesses.
It could develop gradually. At first, they might be cautious around each other. For example, Human Toothless may be a bit shy because he's not used to interacting with humans in this form. The female Hiccup might be intrigued by his connection to dragons. As they spend more time together, like when they are training young dragons, they start to trust each other more. And trust can easily lead to deeper feelings.
During the pregnancy, their relationship would shift to be more centered around the upcoming baby. Hiccup would be constantly thinking about how to keep Astrid and the baby safe, especially with his role as a leader. Astrid would be dealing with the physical and emotional changes of pregnancy, and Hiccup would be her main support. This would bring out new aspects of their love for each other, like a deeper level of understanding and patience.
In fanfiction, their relationship could be further developed by having them communicate in new ways. Maybe Toothless discovers a new form of communication that only Hiccup can understand. This could lead to more in - depth conversations between them. Also, they could have a falling out over something, like a misunderstanding about a dragon's behavior. But through this conflict, they'll learn more about each other's feelings and come out with an even stronger bond in the end.