A really popular one is the Norse mythology crossover. Since HTTYD already has a Viking setting which is closely related to Norse mythology, it's easy to blend. Dragons could be related to the Midgard Serpent, for example. Another popular one is Greek mythology. Imagine a dragon with the power of a Gorgon, able to turn things to stone. It makes for some very exciting and unique fanfictions.
One popular theme is the clash of different mythologies. For example, when Httyd crosses over with other myth - based stories like Egyptian or Celtic mythology. Dragons from Httyd interact with the unique creatures and magic from those mythologies.
One more is 'Httyd and Percy Jackson Crossover'. This one is fascinating as it mixes the Norse mythology elements from Httyd with the Greek mythology of Percy Jackson. The dragons and demigods coming together create a whole new set of adventures and challenges.
One interesting crossover could be the Night King trying to control the dragons from HTTYD. It would be a huge battle between the magic of the Night King and the bond between the dragon riders. Another could be Daenerys meeting the Vikings and learning about their different ways of taming dragons.
The fanfiction might focus on how the relationships between the characters change when the two worlds of HTTYD and HTTYD2 collide. For example, Astrid might have to adapt to new ways of dragon riding she sees in HTTYD2, and this could cause some friction or growth in her relationship with Hiccup. It could also involve the dragons from the first movie learning new skills from the HTTYD2 dragons.
Another option could be Wattpad. While it has a wide variety of content, you might be able to find some great HTTYD2 fanfiction related to HTTYD there. It's important to note that the quality can vary, so you may need to sift through some stories to find the really good ones.
One interesting HTTYD2 fanfiction is 'The Next Adventure'. It continues the story after the events of HTTYD2, exploring new lands and challenges for Hiccup and Toothless. It really captures the spirit of the movies.
Some characters might be amused. For example, Hiccup, being the open - minded and curious one, could find it interesting to see how fans imagine different scenarios. He might even get some new ideas for his own adventures.
Astrid would also be a likely main character. In both movies, she plays an important role in Hiccup's life and the dragon - human relationships. She could bring her own skills and perspectives when the two movie storylines meet. And of course, Stoick might be there too, as his presence and leadership were important in the first movie and his legacy could play a part in the fanfiction that combines the two.
Fishlegs would likely have a very animated reaction. If he comes across some fanfiction that has really strange dragon facts, he'd be so shocked. He might start rambling on about how wrong it is according to his extensive dragon knowledge. His eyes would be wide and he'd be waving his hands around.