Another idea that will probably be nothing more than an idea. It kind of started out because there are plenty of stories where a human finds a hurt dragon and nurses them back to health, gaining their trust and friendship in the process (Hiccup, Heather, etc.). But I wanted a story where a dragon finds a hurt human. So I give you a role swap AU from Toothless's POV.
Toothless is on one of his annual raids on Berk and barely dodges a trap. After such a nasty scare, he fires back and growls in satisfaction as the launcher catches fire and collapses onto a struggling Viking. Toothless lands to finish the job, but he is conflicted to find that it's a young human instead of a scary Viking trapped under the rubble. Still, he thinks of all the dangerous Vikings that were once young humans but grew into murderous killing machines, and Toothless bares his teeth, ready to end this future Viking. But just as Hiccup saw himself in Toothless's eyes, had their roles been swapped, Toothless sees himself in Hiccup's. He drags the human out from under the debris and discovers with a pang of guilt that the young human's leg has been blasted clean off. Toothless debates whether or not sparing the human's life was good enough until an especially scary Viking came yelling, ax swinging. The vast Viking with hair like a roaring fire had obviously come to finish them both off. Toothless couldn't bring himself to leave the hurt human to the mercy of the angry Viking, so he carried the boy to his nook on the dragon island.
But a dragon nest is no place for an injured human, Toothless soon realizes. After a few close calls, Toothless decides he'd better hide his human somewhere safer. He takes his human to a cave in the forest, a ways away from Berk. But things aren't looking good for the little human. He's deathly pale, and when he doesn't wake up one morning, Toothless decides he needs a human healer. Toothless fly's to Berk that night and gently kidnaps the small wrinkly Viking healer, Gothi. Gothi arrives just in time and gives Hiccup the medical attention he desperately needs. After the initial shock of being carried off by a Night Fury and watching Toothless interact with Hiccup (bringing him various sized sticks as potential peg legs, all of which Hiccup gently refuses), she realizes that dragons are kind, compassionate creatures. Still, after a few days, once satisfied that Hiccup will be alright, she sets off to go back to Berk and inform Stoik that Hiccup is, in fact, alive. After all, Berk thinks a Night Fury ate Hiccup. It's only fair they knew the truth, and while Hiccup was doing well, it would be quite a while before he'd be able to make the journey home.
Hiccup doesn't want to let Gothi go and risk endangering Toothless, but Gothi convinces him it's not fair to let Stoik mourn his son's death anymore. So Hiccup agrees to let her go under the conditions they write out precisely what Gothi's going to tell Stoik. They agree on a script, and Gothi leaves. Not long after Gothi leaves, Toothless bumps into another young skinny human, but this one is much feistier than Toothless's current roommate. This particular human even poses quite a threat, and Toothless is forced to go on the offensive. But before things could get too ugly, the small human yells, "For Hiccup (curse his scrawny butt)!" and Toothless adds two and two together. With some difficulty, he scoops up the girl and fly's her the distance to his cave, where Hiccup is sleeping peacefully. Astrid probably punches him and demands an explanation, and Hiccup provides one. Then Hiccup wonders what she's doing all the way out here, and Astrid says there was a massive honor awaiting whoever found the location of the Night Fury's lair (Hiccup is a bit disappointed). Astrid explains that without Gothi pointing her in the right direction, she would've never found them.
Hiccup and Astrid get some bonding time while Gothi travels to Berk, and Hiccup tells her all about dragons, and Astrid starts to get feelings for him. Then one day, while Astrid's helping Hiccup, Toothless growls a warning and flies off to face whatever is coming. Sure enough, a few miles out, Toothless is ambushed by Stoik and practically all of Berk. They pin Toothless down, but just before Stoik can finish the beast, Gothi stops him and writes something like, "If you don't believe me that your son's alive and this dragon saved him, then believe me when I say there's an island with a nest full of dragons, and you'll never find it without this dragon." This convinces Stoik to let Toothless live and use him to find the dragon nest. Gothi runs back to the cave and tells Hiccup and Astrid what happened. They decide they have to do something, so with Astrid's help, they go back to Berk and train the other teens. They fly to the island, and stuff happens as it does in the movie, but Toothless loses his tail fin in the fight.