The ending of 'Kiss Him Not Me' manga ties up most of the storylines neatly. The main character finds love and resolution in a way that's satisfying for fans.
The characters in 'httyd' would likely have complex reactions to the future in the fanfiction. Hiccup, for example, has always been a forward - thinking character. He would likely be intrigued by the possibilities the future holds, whether it's in terms of dragon - human relations or new inventions. However, he would also be concerned about protecting his tribe and the dragons. Astrid would be more practical. She would assess the potential threats and benefits. The dragons themselves, such as Toothless, would react based on their bond with their human counterparts. If they sense that Hiccup is positive about the future, they might be more open to it, but if there are signs of danger, they would be on high alert.
Naruto might react with a sense of responsibility. If he saw a future where Konoha was in danger, he would feel it was his duty to do something about it. He could start by gathering information from the future vision, like the weaknesses of the enemy. Then, he would share this knowledge with his comrades and come up with a plan to protect the village.