The Big Four AU: A Friendship Told Four Ways
Or basically Four Rise of the Brave Tangled Dragons AU where they all exist in each other's universe.
How to Train Your Dragon AU: Hiccup entrusts his secret of Toothless with the only people on Berk he trusts; his friends: Rapunzel, daughter of the village healer. Merida, a training shieldmaiden and the only teen that doesn’t pick on Hiccup. And Jack Frost, a young shepherd boy that guards the flock, or what’s left of it.
Tangled AU: Determined to see the lanterns for her 18th birthday, Rapunzel makes a deal with a trio of thieves: Hiccup the Blacksmith, Merida the Archer, and Jack Frost, the Trickster.
Brave AU: When the four clans joined together, an unlikely friendship was forged between the children of the clans; Princess Merida of Clan DunBroch. Prince Hiccup of Clan Dingwall. Princess Rapunzel of Clan Macintosh. Prince Jackson of Clan MacGuffin.
Rise of the Guardians AU: For 300 years, Jack Frost was unseen by the mortal world, his only friends were other young seasonal spirits like him; Hiccup the Autumn Spirit, Rapunzel the Spring Spirit, & Merida the Summer Spirit.