Spawning from fanart, this AU places the Big Four, and other such characters as archaeologists that search for forgotten kingdoms, cities, tombs and legendary places, treasures (like the Moonstone, the Mirror of Incanta and the Dragon Eye) and undercover myths, legends and how they were made. Even when the magic/witchcraft, monsters, places and gods from those stories are true and uncovering them changes their lives.
Hiccup is portrayed as a archaeologist that studies Viking lore, history, artifacts and legends. He can also be a paleontologist that has his life turn upside down when he uncovered a fossilized dragon (and Viking next to it). Some of the treasures he finds can be Hamish I's long, lost treasure and the Dragon Eye.
Merida is a archaeologist that seeks the ruins and treasures of the White King. She wears one of her discoveries around her neck. She'll uses her archery skills to trigger and locate traps so she won't walk into them and fend off wild animals and those who wish to harm her. Like unfriendly locals that what to prevent her taking their ancestors' treasures from their land, thieves that try to take her "prize" and "findings" for money and some of the beasts that guard ruins and are said to exist in legend(s).
Rapunzel studies and searches for fossilize and surviving ancient plants. She can also be a archaeologist that is searching for the lost crown of sun goddess, that is said to be hidden within a hiding tower.
Tink's journey in the Lost Treasure can easily make her treasure hunter or archaeologist that searches for the Moonstone and the Fall Scepter, so she can find the location of pirate treasure that lies in a ship that sunk, but never sank and a mirror that can lead her to other lost and legendary treasure and the place it originally came from.
Professor Radcliffe Bomba believes in the legends of the ancient, lost city/kingdom of Moonhaven and it's Green Knights and he hopes that he may someday find, explore and see it with his own eyes. His daughter promised to go with him when he believes that he has finally finds the location, so he'll have someone to cheer him when he realizes that it's just a "legend".
Instead of dragons Viggo Grimborn and his Dragon Hunters would be Treasure Hunters and Grave Robbers, that rob tombs, temples and even sunken ships of their treasures and sells them on the black market. The Dragon Eye would be the prize of Viggo's privet collection, as it belonged to his ancestors and is a map to other treasures that he could sell.