One great drago x hiccup fanfiction is 'The Unlikely Bond'. It delves into how Drago and Hiccup might form an unexpected alliance. Another is 'Drago's Redemption', which shows Drago's journey towards changing his ways in relation to Hiccup. And 'A New Beginning' where they start to see each other in a different light.
Toothless is the most likely one. Their bond is so strong that Toothless would sense Hiccup's distress and come to his rescue. He could fly into Drago's hideout and cause chaos to free Hiccup.
First, understand the characters. Know Drago's tough and often cruel nature and Hiccup's more empathetic and intelligent side. Then, come up with an interesting plot. Maybe it could be about them getting stranded together and having to rely on each other. Another key is to write good dialogue that shows their different personalities clashing or coming together.
In this kind of fanfiction, perhaps Hiccup would be in great danger. Drago might use Hiccup as a hostage to get something he wants, like controlling the dragons that are allied with Hiccup's people. He could also try to force Hiccup to reveal the secrets of his unique bond with Toothless.
Hiccup's friends would be extremely worried. Astrid, being Hiccup's closest friend and love interest, would probably be the first to take action. She would gather the other riders and start planning a rescue mission right away. Snotlout might initially be a bit hesitant, but the thought of losing Hiccup would push him to join in. Fishlegs would use his knowledge of dragons to figure out where Drago might be hiding Hiccup.
One possible plot twist could be that Drago kidnaps Hiccup to use him as a bargaining chip against the dragons. Maybe he wants to force the dragons to submit to his will by threatening Hiccup's life. Another could be that Drago discovers some special power within Hiccup that he wants to harness for his own evil purposes. For example, Hiccup's ability to communicate with dragons might be something Drago wants to use to control all dragons.
Their relationship can be developed through shared experiences. For example, they could face a common enemy together. This would force them to rely on each other, strengthening their bond.
One idea could be that Drago and Wavern embark on a journey across a magical land to find a lost artifact that has the power to save their kind. Along the way, they face various challenges that test their friendship and loyalty.
To develop Drago's character, you could focus on his bravery. For example, show him facing his fears in different situations. For Wavern, emphasize her intelligence. Have her solve complex puzzles or come up with clever plans. You can also give them both flaws like Drago being a bit too impulsive at times and Wavern being overly cautious in some cases.