About this AUEdit
In this AU, the Big Four exist within the same universe as the Cartoon Network series Steven Universe.
Here, they could either be humans or Gems - a magical alien race of polymorphic gemstones (their Gem equivalents being "Gemsonas" of the characters). They usually play as members of the Crystal Gems, a group of Gems who rebelled against the Gem home-world and became the guardians of Earth and the humans that live on it.
Should he be a Gem, Hiccup is often portrayed as a green-colored Gem, "his" weapon being a Gem-based version of his fire-sword Inferno. In other circumstances, "he" would instead wield a Gem-based equivalent of his ornate gronkle-iron shield. Because Gems are capable of many scientific achievements thousands of years ahead of humanity, Hiccup would be adept at the technicals of advanced technology Gems possess much like Pearl, making repairing and producing Earth technology (from cars to weaponry) child's play in comparison. In the case of Toothless, odds are he would be a Gem-based animal very similar to Lion. All in all, Hiccup would be very much like Rose Quartz, and is often placed in the same role as her.
Being (debatably) the most normal of the Big Four, Hiccup is also likely to be a normal Human, acting as a friend and/or love interest to the protagonist much like Connie.
Should he be a Gem, Jack is often portrayed as a blue-colored Gem, "his" weapon being a Gem-based version of "his" shepherd's crook. Much like the original Jack Frost, his gemsona would have powers relegated to ice or the cold much like Sapphire.
Should she be a Gem, Merida is often portrayed as either a red or green-colored Gem, her weapon either being a Gem-based version of her bow and arrows or a sword (the symbol of the Clan DunBroch and a weapon Merida is more than capable of using). Should it be the bow, whether or not it would operate like Opal's bow depends on the story.
Should she be a Gem, Rapunzel is often portrayed as either a pink or yellow-colored Gem, her weapon being a Gem-based version of her frying pan. Much like Rose Quartz, she would most likely possess healing powers and abilities related to plants (given their names) or - more likely - the sun.