Hiccup and Magnus meet in Valhalla in a fight to the death. All of the HTTYD gang are on the floor above Magnus's so their groups like to team up for battles. Maybe they go on an adventure and find Toothless's future family in the Nine Realms. They stop Loki from using the dragons during Ragnarock. Maybe? I don't know. I just think it would be cool to see Hiccup, Magnus, and Alex being awesome together.

(Yes I know a lot of people hate that show. I have the privilege of seeing it through the eyes of a six year old. The animation is bad, but the story and the connections are ok.)