Hiccup's death in the fanfic could be a shock and create a lot of tension. It might force the other characters to step up and take on new roles, or it could even serve as a turning point for the entire story's theme and message.
Often, in such fanfictions, Hiccup might die in a heroic battle against an ancient evil that threatens the entire dragon world. This could lead to a powerful emotional arc for the other characters.
It can have a very significant impact. Hiccup's death might trigger emotional responses from other characters, leading to new plot developments and character growth.
Well, Hiccup is very resourceful. He could use the environment around him to his advantage. If he is locked in a place, he might find a weak spot in the structure, like a loose board or a hole in the wall. And he could call for Toothless secretly. Toothless, being highly intelligent, could find Hiccup and rescue him. Also, Hiccup could befriend some of Viggo's minions and convince them to help him escape.
Toothless is the most likely one. Their bond is so strong that Toothless would sense Hiccup's distress and come to his rescue. He could fly into Drago's hideout and cause chaos to free Hiccup.
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.
Toothless is definitely one of the most popular. Since he's Hiccup's closest dragon companion in the original story, in fanfics where Hiccup is a shapeshifter, Toothless can have a whole new dynamic with him. He might be the one to help Hiccup understand his new abilities, or be protective of him when others are suspicious. Astrid is also popular. She could be the one who discovers Hiccup's secret and has to decide whether to keep it or tell the others. Her relationship with Hiccup, which is already complex in the original, can take on new twists and turns in these fanfics.
Hiccup's friends would be extremely worried. Astrid, being Hiccup's closest friend and love interest, would probably be the first to take action. She would gather the other riders and start planning a rescue mission right away. Snotlout might initially be a bit hesitant, but the thought of losing Hiccup would push him to join in. Fishlegs would use his knowledge of dragons to figure out where Drago might be hiding Hiccup.
Hiccup could try to resist by using his intelligence. He might find loopholes in Dagur's plans or demands. For example, if there are certain cultural or traditional rules that Dagur is using to force the marriage, Hiccup could find ways to show that those rules are being misinterpreted or misapplied.