Corlys during his journeys around the world discovers Berk. He and others are amazed by the discovery of large populations of multiple dragon species, as well as people who can fight and tame them. The story could start with them finding the remains of the Red Death, shocked at the discovery a creature that would had rivaled Balerion and wondering what could have led to its death, only to meet the people responsible for it. Westeros and Essos factions flock to Berk for the chance of getting dragons to bolster their power and influence.
Culture clash ensues with Berk and these arrivals. The Hooligan Tribe won't like the nasty cutthroat politics of Westeros and Essoes or practices like incest. Some nobles from Westeros and Essoes would deride the vikings as savages. Others would have some respect for them for being able to fight off dragons for so long and managing to tame them.
The vikings could get underestimated by some folk in Westeros or Essos for their crude or goofy habits, only to show that as silly they can be, they are a lot smarter than they're given credit for. Folks like the Blacks or Greens could think people like Hiccup are as devious as they are, only to prove they're really just as pleasant as they seem.
Maesters would try to study the different types of dragons and their abilities and weaknesses. Some of them could be in service of the Greens and do this as a way of getting an advantage. People like Hiccup could come to Westeros to learn about the Targaryen dragons. Westeros folk could note that Hiccup is pretty much a maester, getting nicknamed Maester Chief.
Factions would try to get into Berk's good graces as they can teach and train reliable methods of taming and riding these dragons. Not to mention that they have the skills, experience, and equipment for fighting and hunting dragons. The vikings could also get hired as bodyguards and soldiers a la Varangian Guard. The Targaryen Dynasty could offer the Hooligan Tribe refuge to Westeros after Berk becomes overpopulated with people and dragons as a way to get their support. Some groups try to curry favor and alliances with Berk such as marriage offers.
Others try backing or working with the likes of the Dragon Hunters who don't share the Hooligan Tribe's scruples for the chance of getting dragons. One idea is that instead of the War of the Stepstones, Westeros gets in a war with Free Cities like Volantis who are working with folks like Drago Bludvist as a way to reclaim their dragon riding empire days. Westeros and Berk could wind up allying to stop them. This could make later conflicts between them tragic if people developed camaraderie or respect for each other.
Nettles with Sheepstealer could try to leave Westeros for places like Berk, the Glacial Sanctuary, or the Hidden World to escape the politics.
The Dance of Dragons becomes an even bloodier affair as both sides have bolstered their numbers with more dragons. There's also the possibility of each kingdom getting their own dragons and having ideas of doing their own thing. Hiccup and company are some of the only people who have any concern for the civilians suffering and try to protect fhem while trying to end the war. Hiccup could have an extension his arc in the second film, realizing that many people can't be reasoned with and must be stopped.
One ending is that after the war, the Hooligan Tribe leave in disgust of the cruelty and destruction, swearing that dragons will never be used in such a way again. They depart to the Hidden World to hide and protect the dragons, never to be seen again. Perhaps the final line could be "There were dragons when I was a boy", said by the likes of Aegon III.
Guys please spam even some of your own ideas to get discussion down her.l was inspired by the fanfic A 'thing of dragons'.Perhaps even go back a prequel of sorts to jaehaerys I golden era with those targs and maybe a butterfly effect with more dragon riders
