My idea is that a httyd fan who just started the first episode of Dragons the nine realms, maybe he finished the episode or he finished the first season and was about to start season 2 I don't know up to the author who decides to write this story but he is reincarnated or transmigrated whichever with his own dragon during Hiccups time so he interacts with them for sure so you could write about how our httyd fan interacts with Hiccup and the gang from the first movie all the way until the end or you can kind of skip to the end where the mc is saying his goodbye to Hiccup and the gang and travels to the hidden world or the mc was put into the world near the end of the last movie so he just travels to the hidden world and locates what will be called the crystal realm by the kids in Dragons the nine realms and uses something or has the ability that allows him and his dragon to be put in stasis or sleep until start of the Dragons the nine realms series start so yeah whoever writes this story can have it encompass both Hiccups story and the nine realms or can just focus on the nine realms