(COMPLETED, BUT UNDER REVISION AND EDITING. Yeah, it's one of those runaway AU tropes; I have a soft spot for them. COVER made by @spiritofpeace.)
Hiccup and Toothless left Berk thinking no one cared.
Being the neglected child of Berk (despite his family, and being the heir to the village), Hiccup decided he'd had enough. He took Toothless and his most treasured belongings and high-tailed it out of there.
When he runs into his long-lost mother, life looks up for Hiccup. He's found a place where he belongs.
Meanwhile, on Berk, the village assumed Hiccup was dead by a Night Fury's doing and left it at that. Astrid and her friends, however, didn't think such. When a surprise trip overseas and an unfortunate turn of events allows the Berkians to find an odd nest in the middle of nowhere, they may prove that the lost heir to Berk isn't as dead as everyone thinks.
(I do not own HTTYD in any way, shape, or form. I'm simply one of those nerds who decided to write something about it.)