I am sitting here thinking about how modern riders would be like (ignoring the Nine Realms for a second). I'm thinking, what if the dragons resurface due to natural disasters related to global warming and humanity being thrown in a panic.
Of course, humanity immediately try to subjugate them and in that course a lot of dragons die. However, some people start bonding with their dragons.
These bonds eventually lead to a divide where some believe humanity needs to return to a sense of nature, while others want to use dragons for war, and the third group wants to treat them as animals or pets no different from lions and dogs.
I feel like through the course of this series the characters learn more about Berk and the old world by finding hidden artifacts where berk would have been. Maybe a small village in the outskirts that still tell the stories orally albeit skewed.
Maybe we get to explore the legacy hiccup left behind, the good and the bad, and how his choices have effected modern riders and the entire world.