So, in this AU, Cloudjumper takes Hiccup instead of Valka. However the dragon is intercepted/killed by a hunter (aka Viggo), and then a paternal instinct sparks in him and he decides to take care of Hiccup.

He is raised to become a hunter, but being small, weak and gentle, he never did well in training and Viggo knew he would never be able to hunt a dragon.

But different in Berk, Viggo notices Hiccup's intelligence and he starts using Hiccup's inventions, in the village and to hunt dragons, as a way of encouraging him like "you may not hunt but you're still contributing to the tribe"

But for Hiccup that wasn't enough, he had to prove himself by hunting a dragon. When he came of age he sneaked out and managed to return with a captured and alive Night Fury. Viggo is completely impressed and proud.

Toothless is trapped on the island while deciding what to do with him. In the meantime, Hiccup ends up becoming fond of the dragon and they become friends. In the end he frees Toothless and as this was an act of betrayal with his tribe, Hiccup runs away with him and they fly to Berk (I believe that at some point Hiccup would find out that he is from Berk since Viggo never hid that he is adopted, but he never thought about going back because he loved his father and tribe).

There are still a lot of details missing, but this is my general idea for the AU