Well, for a httyd transformers crossover fanfic, start with a strong concept. Maybe something like the Autobots land on Berk and have to figure out how to deal with the dragons. Focus on the interactions between the characters. The vikings' reactions to the giant robots and the Transformers' views on the dragons would be really interesting to explore. And make sure your writing style is engaging, using vivid descriptions to bring the crossover world to life.
First, you need to have a clear concept of time travel in the HTTYD universe. Decide if it's a one - way trip or if there are rules for coming back. Then, think about how the characters will react to this new situation. For example, Hiccup might be both excited and a bit scared about exploring a different time period. You can also introduce new dragons that were extinct or not yet discovered in the original timeline. And don't forget to keep the essence of the HTTYD story, like the bond between humans and dragons.
There is a 'brave httyd fanfic' called 'Alliance of Heroes'. In this fanfic, the main characters from 'Brave' find themselves in Berk and team up with Hiccup and his friends. They face a common enemy and it's really interesting to see how their different skills and personalities blend together in the story.
Yes, there are likely some popular ones. For instance, some fanfictions that explore the relationship between Hiccup and Astrid during the time leading up to HTTYD 2 are quite popular. They might show how their relationship evolves and how they support each other in anticipation of the new threats in the second movie.
There are fanfics that center around the growth and development of anthro dragons. They might start as outcasts in their own world and gradually find their place. These stories often involve character - building and the exploration of friendship. Some might also include elements of mystery, like uncovering ancient secrets related to the anthro dragons.
Another theme is self - discovery. The child in the fanfic is usually on a journey, both literally and metaphorically. As they interact with the dragon and other characters in the How to Train Your Dragon world, they learn more about themselves, their strengths and weaknesses.
One idea could be that the dragon's fire power can be used to recharge the Transformers in some way. Another is that the Autobots could learn the concept of flying from the dragons and incorporate it into their own designs. Or perhaps the Decepticons try to capture the dragons to use their power for evil purposes, and the vikings and Autobots have to work together to stop them.
The fanfic could focus on Astrid's pregnancy journey that led to the birth. It might detail how she coped with the physical changes and the support she received from Hiccup and her friends. There could be a part where they prepare for the baby's arrival, like building a nursery or gathering supplies. After the birth, it could show the couple's joy and the celebration within the village, with everyone coming together to welcome the new life.